Being a portable device, one that has the inputs and technical capability of running games we've previously only been able to experience on home consoles, one would think any good game would be worth playing to someone who is interested in a handheld experience. I know some genres and game types are automatically preferred on handheld by many, like platformers, puzzle games, indies. Mobile games have also made it so people are used to gaming when it is convenient for them.
Vita has some decent exclusives as well as upcoming games such as Killzone and Tearway already announced that are set to be some of the best games in their genre on handhelds. It also has some great handheld exclusives that are otherwise available on less portable devices, and usually the best multiport version too.
The thing that people forget is exclusives for the Vita cost a lot of money. The games need to take advantage of the hardware and seeing as it is closer to the PS3 than a PSP in what it can do, it becomes really expensive to make new games and completely and not have so many ports and sequels. Despite what people may claim, the caliber of games at the very least on a technical level but also about potential for immersion is different between the handhelds. There are of course exceptions from talented developers but generally speaking, it's why we get upgraded consoles to begin with. We wanted things not possible on the PSP.
Especially near the beginning of the Vita's life, before the PS3 has been phased off - it doesn't make much sense for third party publishers to release exclusives as they'd be essentially competing with their own sales on the larger PS360 userbases. But as time passes and people move onto next-gen consoles, those PS3 quality games will be appealing at a lower price and more convenient form on the Vita for both consumers and publishers. Especially mid-tier ones (which is the majority now).
As the Vita userbase picks up following world-wide price-cuts and advertising pushes alongside the PS4 (bringing mainstreams focus back to gaming) the forced remote play support from PS4 to Vita will mean that ports on Vita will make less sense. As people with PS4s (quite likely the majority with the Vita demographic eventually, especially with the support itself - it makes next-gen portable!) will already have that available essentially and it would make more sense to make a spinoff/exclusive.
Give it time but don't expect miracle exclusives from third parties unless Sony has had heavy involvement in negotiating the deals for now.