DY: 1. Rosh Yeshiva in EY tells talmid not to go to BMG.
That just shifts the question to the RY.
Maybe….but Joseph didn’t ask that. He asked why a certain bochur would choose another option. His RY told him not to.
You also wrote: “but I think the number 1 reason would be wanting a smaller yeshiva, with athe opportunity to gain a kesher with the rosh hayeshiva.”
I have personal experience with a certain RY in Eretz Yisroel (who has a reputation for knowing every bochur in his yeshiva) who prefers his talmidim not go to BMG due to its size. He feels that a smaller yeshiva will help the bochur be able to develop with the Rosh Yeshiva / Maggid Shiur.