The theory as to why Charlotte was going for the NXT title rather than a “main roster” one is that WWE wants to give NXT ratings a bump, but most assumed any build to a match wouldn’t take place until after this weekend’s NXT TakeOver: Portland show.
However, the build has already started and according to Dave Meltzer on
Wrestling Observer Radio, the reason is more to do with WWE not expecting to be so resoundingly beaten by AEW in the Wednesday Night Wars in January 2020.
“They’re trying to stall it out. The whole idea is to build up NXT ratings with Charlotte Flair. They didn’t feel like waiting [until after WrestleMania]. They got blindsided by the January ratings. They were not expecting that. They thought that the corner had been turned.”
So it appears the decision to have Charlotte win the Rumble was specifically to try and give NXT the ratings rub, and starting the feud as early as possible was for the same reason.