My guess is that Vince saw her as the way to legitimize the division while needing the least work to do it worth. And he's not entirely wrong for thinking that. HHH did a similar strategy in NXT.
If you look at her NXT reign, it was more NXT booking and her opponents that got her over. Her fist big moment was the NXT title win. She got to go over the best main roster female wrestler and NXT built a great Flair vs. Hart stroryline with both legends at ringside. That was in May of 2014 and nothing really notable happened until Sasha enters the scene in December (there was a forgettable TakeOver title defense against Bayley in Sept).
Your wrong that she made her Raw debut this past summer, it was actually Dec. 8th 2014. She had a non-title rematch against Natalya to hype up her upcoming TaekOver match against Banks. She came out with the NXT title and got a lengthy video package putting over her run in NXT (sidenote that these are more things to add to the list, especially compared to Sasha who unlike the other champions was never allowed to wear her belt out to matches and only got a video packager after Respect). Flair also started showing up in the Tapout commercials around Mania.
She gets the big push after the callups but honestly, looking at the situation, I'd rather it be her than the others. Look at Paige's big push. She's over, but not because she's a good wrestler. It wasn't even a year ago that she's part of the infamous 30 second match. She is however a star of Total Divas, wearing more make-up than ever and asking for that stupid belt as her Christmas wish.
Charlotte was just next for the old processing line of turning the women into Total Divas (she did just wrestle as Mrs. Claus). She's been the sacrificial lamb while they slowly figure out they cannot serve both Total Divas and NXT audiences at once. And thanks in large part to some Boss ass performances in Aug and Oct, main roster creative is slowing learning that NXT may not be so 'niche' after all.