What would prompt or be a reason a bochor would choose a different Beis Medrash at that age?
Because BMG would have been completely wrong for me and had I gone there, I would likely have ended up completely off the derech. (Yes, I know that some of you think that I am that way now anyway, but that’s your opinion, not mine).
And who are the most popular alternative Yeshivos for them
The one I went to was a small beis midrash and clearly not one of “the most popular alternatives.” But it was right for me in ways that BMG never could have been.
The Wolf