Identifying the reasons for Vita's struggles seem very challenging for most because it's such a hardcore focused device that very few people like to find fault with it.
Nobody wants to blame the hardware, but few want to blame the software, either. Many people want to argue that the hardware is amazing and the lineup is great, but one of those things has to be untrue for the system to be selling like it is. From an objective perspective of course; you're perfectly allowed to subjectively love the Virtua Boy's lineup, if you so desire.
In these sorts of situations, people tend to place all the blame on advertising, because it allows the product to escape unscathed; it isn't the system or the game's fault, it's just that Sony (or Nintendo or Microsoft) aren't properly communicating how amazing and awesome this system and its games are. I don't mean to suggest advertising never matters at all, but people disproportionately rely on this explanation because it allows them to avoid blaming the system. In the case of Vita, the sales have been so terrible that something else must be going on.