why is she not interested in Lenny Bruce?

Miriam still loves Joel. Joel still loves her too. That's how I see it.

It's better for their relationship to remain Platonic imo.

I believe that she just considers Lenny a mentor and a friend. She respects Lenny's skill and he respects hers. He has been her knight in shining armor a couple of times, and to me, it is nice that he being so good to her. Considering how his story plays out, I hope we see a bit more of him before his fate catches up to him.

I don’t understand her either. This actor is hot but I think he makes fictional Lenny a more knight in shining armor, mystery man character than the real Lenny Bruce. I guess the writers want to respect the memory of Lenny and not take too many liberties with his character and storyline in this by adding fake affairs.