Several reasons:
1) Her personality is boring as hell, or at least her online persona is, and social media is one of the biggest tools celebrities have to use these days. The industry has changed in a way that labels and managers rely on the internet for a lot of promo. The only stars who don't really have to have big online presences are the ones who garnered their fanbases before the internet was around, like Britney, Beyonce, etc. Otherwise, unless you have a gigantic support from your label, new artists need to use the internet to their best ability. Ariana is very good at this, and she got her huge following by posting covers on her Youtube channel and being very open on Twitter. Victoria presents herself as a boring person. She might not realize it, but she does. No one is going to stan for someone like that. Ariana lets fans feel like they know her and is generally more fun, so people get more rabid about supporting her work and defending her.
2) She doesn't shine in the talent department. If she were an amazing actress or singer, it might make up for her lack of charisma, but she isn't. Girl is mediocre to the bone.
3) Nickelodeon (her label) isn't invested in making her an "it" girl. Nickelodeon and Disney are totally different. Nick only cares about making successful television shows. Disney cares about not only making successful shows, but in turning their female stars into pop sensations and household names. That's why every time you saw a new Disney starlet with a show, you could count on having to see her face everywhere for a long time. Nick doesn't invest in their stars like that. The furthest their music stars will go is maybe getting a single in the top 50 on the Hot 100 like Miranda Cosgrove. They clearly don't care about making Victoria happen, and if you don't have strong label support, it's not possible to be successful.
4) Her feud with Ariana did her no favors. And yes, she is the bully Ariana is referring to. It's NOT Daniella and it's obviously not Liz either. Daniella's been very supportive of Ariana. She's posted a bunch of videos of herself singing along and dancing to The Way. She was at
Ariana's party after it was released.
She just tweeted a few days ago about buying Ariana's album. Both Ariana and Daniella have referred to each other as close friends. The only person on the Victorious cast who has been completely silent about Ariana's career is Victoria. She hasn't acknowledged it at all. No telling people to buy her songs or album, no congratulations on any of Ariana's accomplishments. Ariana was the only person from Victorious that Victoria didn't invite to her birthday party, and she ignored
Ariana's happy birthday tweet. Ariana attempted to bury the hatchet after their Twitter fight by apologizing publicly, but Victoria has continued to be bitter. There has probably always been jealousy there. It was a bad move to get into a fight with a more popular cast member, and now that Ariana is blowing up and she continues to be mad, it just makes people not like her.