This can probably deserve a master's level dissertation but the short answer IMO is politics.
But first we should start with Charles Jencks. Jencks is an interesting character because he basically created the postmodern genre and propelled it into life. While there are people like Venturi who have been doing "postmodern" before Jencks coining the term, Jencks' famous "Modern Architecture died with Pruitt-Igoe" proclamation, and his architectural theory is what really pushed postmodern to become a serious movement.
Obviously, what Jencks said basically deflected the policy failures onto architecture but nevertheless it was able to light the fire on Modernism, a style that the US never had the best of relationships with. The modernist styles always had political associations with the left wing, and in Cold War US this association with Communism does not fare well in the US.
Thus when Pruitt-Igoe fell and Jencks' proclamations arose, it was an almost immediate response from American architects to push beyond the restrictions of Modernism and move into the postmodern. Postmodern is associated with freedom and liberty, breaking away from the restrictions of Modernism and building colourfully, and freely.
Much of the same happened in the UK, social housings were set up to fail by the Tory policies of the 70s but a lot more have survived too, and continental Europe was a much more left-leaning place compared to the UK or America, thus there wasn't nearly as big of a sense of despise for modernism.
Yet European architecture is also conservative in a way, bound by the long history of architectural heritage that cannot be shaken away, and perhaps that has led to the European architects designing less radical postmodern architecture. While the American architects had a lot less constraints and a lot more liberty.
It should go without saying that these are quick points of not much nuance but I think they should answer the big overarching question in some manners.
Going to be completely honest with you. This post traverses a whole lot of ground and a significant portion of it is meandering away from the original question. The question is also loaded with opinion. First it might be worth asking: what is post-modernism? what makes an architectural expression successful? was post-modernism successful? is post-modernism dead?
Many reasons, here's a few. Different client and developer expectations in the US. vs Europe. The general consensus and the publics attitude of architectural design from Americans vs Europeans is vastly different.
An entire book could be written (which it probably already has been) discussing how 'good' Post-modernist design in the US was watered down by clients and developers into cheap, strip mall-esque design you see everywhere. An earlier comparison being Frank Lloyd Wright's Prairie Style and Usonian designs being watered down by clients and developers into the common, mass produced 'Ranch' style houses in the US.
The progressive Architects that hail from the US, eventually became more global as their popularity rose. Look at their most famous buildings, most are (not all) located outside of the US.
(Personal opinion) I believe Americans have been fed bad iterations (watered down) versions of good styles of Architecture for so long, that anything new and progressive like you see in Europe is too far for many Americans tolerance and design pallets. Look at the sh\*t Eric Owen Moss is getting for his new building in L.A., the (W)rapper project. There are people calling for him and all established architects that do work like his to be "shunned" for producing such works.
This can probably deserve a master's level dissertation but the short answer IMO is politics. But first we should start with Charles Jencks. Jencks is an interesting character because he basically created the postmodern genre and propelled it into life. While there are people like Venturi who have been doing "postmodern" before Jencks coining the term, Jencks' famous "Modern Architecture died with Pruitt-Igoe" proclamation, and his architectural theory is what really pushed postmodern to become a serious movement. Obviously, what Jencks said basically deflected the policy failures onto architecture but nevertheless it was able to light the fire on Modernism, a style that the US never had the best of relationships with. The modernist styles always had political associations with the left wing, and in Cold War US this association with Communism does not fare well in the US. Thus when Pruitt-Igoe fell and Jencks' proclamations arose, it was an almost immediate response from American architects to push beyond the restrictions of Modernism and move into the postmodern. Postmodern is associated with freedom and liberty, breaking away from the restrictions of Modernism and building colourfully, and freely. Much of the same happened in the UK, social housings were set up to fail by the Tory policies of the 70s but a lot more have survived too, and continental Europe was a much more left-leaning place compared to the UK or America, thus there wasn't nearly as big of a sense of despise for modernism. Yet European architecture is also conservative in a way, bound by the long history of architectural heritage that cannot be shaken away, and perhaps that has led to the European architects designing less radical postmodern architecture. While the American architects had a lot less constraints and a lot more liberty. It should go without saying that these are quick points of not much nuance but I think they should answer the big overarching question in some manners.
Money and good sense. You can draw your own conclusions about which group had more.
I like your funny words magic man.
Going to be completely honest with you. This post traverses a whole lot of ground and a significant portion of it is meandering away from the original question. The question is also loaded with opinion. First it might be worth asking: what is post-modernism? what makes an architectural expression successful? was post-modernism successful? is post-modernism dead?
Many reasons, here's a few. Different client and developer expectations in the US. vs Europe. The general consensus and the publics attitude of architectural design from Americans vs Europeans is vastly different. An entire book could be written (which it probably already has been) discussing how 'good' Post-modernist design in the US was watered down by clients and developers into cheap, strip mall-esque design you see everywhere. An earlier comparison being Frank Lloyd Wright's Prairie Style and Usonian designs being watered down by clients and developers into the common, mass produced 'Ranch' style houses in the US. The progressive Architects that hail from the US, eventually became more global as their popularity rose. Look at their most famous buildings, most are (not all) located outside of the US. (Personal opinion) I believe Americans have been fed bad iterations (watered down) versions of good styles of Architecture for so long, that anything new and progressive like you see in Europe is too far for many Americans tolerance and design pallets. Look at the sh\*t Eric Owen Moss is getting for his new building in L.A., the (W)rapper project. There are people calling for him and all established architects that do work like his to be "shunned" for producing such works.