A theater might be worse than a baseball stadium. Maybe I could hear a Ta’ana that modern-day sports are not Moshav Leitzim or any of the other Issurim mentioned in the Gemara. I always found making these Siyums and the Asifa in stadiums strange. R’ Moshe and the Tzitz Eliezer both have T’shuvos absolutely blasting making Minyanim in movie theaters. I assume that the Gedolim now think that a sports arena isn’t as bad and therefore the ability to host so many people outweighs any Halachic issues that may or may not be relevant. A theater, though, still feels much more problematic.
(I have no idea who was behind this. I probably don’t want to know. I am sure that they have a very qualified Rav who endorsed this and while if it was up to me I probably would not have, I am sure that whoever decided to do this is far more qualified than I to determine if it’s okay in this circumstance.)