Not to mention that in VTA you really are there to study and do actual learning stuff as a content creator/streamer/what have you, and it has a far longer time to do before you 'graduate' and be a vtuber, and even then you still have no clue whether you could be niji or not. It's not a clear sure-way to join Nijisanji at the time, especially since people don't know if Niji still have regular typical auditions or not.
So when you really want to be a niji vtuber and don't want to go that hassle, you start looking at alternatives. Like the EN auditions.