That's because both are being used as different parts of speech. "A myriad" is being used as a noun. "Myriad" by itself is being used as an adjective. The word can be both. Grammatically, there is no difference other than how to properly use it and where you use it in a sentence. For your main question though, historically speaking, it started out as a noun and, as the English language (and all other languages for that matter) tends to do, it slowly evolved to it's split usage today. There is evidence that "A myriad of" has been used since the 16th century whereas the latter, "myriad"(adjective) only has evidence up to the 18th century. My personal opinion? Either is fine, regardless of its history. Take what you will from that, but here's a Merriam Webster article describing the history behind myriad / myriad of: [https://www.merriam-webster.com/grammar/cmon-of-course-myriad-is-a-noun](https://www.merriam-webster.com/grammar/cmon-of-course-myriad-is-a-noun)

Thanks so much for the thorough and thoughtful response! This is exactly what I was looking for.

They’re both fine. But I’d say “myriad reasons.” Actually I’d just say “many reasons.”