Sony Worldwide Studios president Shuhei Yoshida, in a statement to Gamasutra, is troubled by the industry's response to the PlayStation Vita. Hardware sales of the handheld have fallen short of their expectations, even in Japan, in part due to its high price point and lack of install base.
Some third-party publishers and developers have been looking at these relatively low numbers and have decided to show their support elsewhere or later. Of course, that's a self-fulfilling prophecy: If they don't show support now, then the handheld will continue to stagnate (and perhaps there will be no later). Shuhei Yoshida hopes that this line of thought will change and that third-party support will grow:
One thing that was surprising and disappointing to us was the [lower] number of third parties to come out [in support] after launch.
...In retrospect, there are so many options for publishers now that we cannot take it for granted that our new platform would be supported by third parties, like [it would've been] many years ago.
There are limited resources that third party publishers have, and they have to diversify into new areas constantly; that's a challenge to get the support that we want.
We've been working harder with our third party relations department to secure more content for PS Vita... We are confident that we have the right hardware platform that we have with PS Vita.
With the gaming industry (finally) on the cusp of next-gen console, developers have been needing to decide where to focus their time: Wii U, the next Xbox and PlayStation consoles, 3DS, mobile, social games, Kinect, etc. The Vita is just one of their many options. I don't think the PSPGo helped matters, either.
Luckily, several notable third-party titles are already slated for release this holiday season, like Silent Hill: Book of Memories, Street Fighter X Tekken, Assassin's Creed III: Liberation, Need for Speed Most Wanted, Persona 4: Golden, PlayStation All-Stars Battle Royale, and Ratchet & Clank: Full Frontal Assault. Of course, only two of them can be considered as exclusive IPs for the Vita, but still...