You don't really have to have Asuka on the main roster. In just the last little while you had Dana (for what she's worth), Nia Jax, Bayley, Carmella, and Alexa Bliss all debut and are new. Asuka coming up now could just be white noise - another new woman - when they already have so many, or maybe (probably) they would have to put her over them no matter which brand they are on. Very probably, she would have to win the the title (regardless of the brand) either on her first night, or at her first PPV, and at this point doing that wouldn't serve anyone other than herself. Right now that spot is open for her to make a bigger impact upon her debut down the line.
They have to have Asuka on the main roster because she will significantly add to the quality of the roster. Asuka would only be white noise if you assume that she's little different from anyone else on the roster when she is very obviously not. She is very unique, very good, and very entertaining and also something that helps further to legitimize the women's wrestling as a whole. And yeah, they would have to hotshot her to the main event but that doesn't mean it would have to be overnight nor does it mean it couldn't benefit anyone but herself. You debut her on Smackdown right now, she can go right down the line from Natty to Carmella to Bliss to Naomi to Nikki before they get to Becky. That's at least five months worth of singles programming for Asuka before even getting to the title picture if they wanted to.
It doesn't matter when she debuts in the sense that she's going to make a huge impact regardless. Whether that's this week, next week, next month, or next year. But you need your best talents working your top shows and there is nobody better than Asuka under their umbrella in the women's division. Certainly a few that can be argued as her equal but none that are better and plenty up right now that don't deserve to be there.