Bayley was too big at NXT to be a brought up as an underdog as you can't have someone presented as important and at the same time be the underdog. Bayley always had to be a big thing from the start.
And it's weird because they DID make her seem like a big deal early on. When she showed up at Battleground, and then debuted on Raw, the commentators played it up like a big deal that she was there (as did Mick Foley). And they repeatedly referenced her being a "former NXT Women's Champion." And it was pretty clear by the pops that she got that a lot of the audience knew who she was.
But then they just dropped that after a while and went back to trying to rehash her underdog story from NXT (lazy/uncreative writing there), and then managed to screw that up as well.
Also they seem to want to pick and choose what they acknowledge from NXT. They reference her title run, and her dynamic with Sasha. But then they ignore that she made it to the top of the division. She beat Charlotte, she beat Sasha, she beat Nia, etc. And when she lost, it was to Asuka, who NO ONE has been able to beat one on one in over a year and a half now. But all of that is brushed aside on Raw to try and do the "underdog story" yet again.