Preparing for the future.. Vita is just a device with now outdated mobile parts.. Soon they all go full mobile..

Sony tricked Japanese publishers into signing a 3-year contract swearing exclusivity.

No one knows why. It's like... Why are people buying PS4s on hype alone? Nobody knows.

Cuz the Vita is like the Kira from Death Note. Even if the Vita died, the followers will still cling to hope for their savior to return x )

Ryng, I was *here* on VGC after the Vita's launch. I know what people were expecting, and nobody expected it to get this level of support in Japan after the awful first few months. 

Probably had to do with fanbases. By that I mean many of the low-mid tier franchise had fostered a following on the PSP and that audience would naturally transfer to the vita. So it made sense for developers to make these games for the PS Vita.

All the mainstream stuff was more suitable for the better selling 3DS.

It's similar to why Sonic, skylander, disney infinity and rayman games keeps coming to Nintendo consoles.

That's my explanation.

Since most other people have offered nice sensible answers (smaller dev support; lots of cross-platform development due to ease of porting; cross-gen stuff) I'll just address all the examples that you listed in the OP:

Sony assisted in development & advertised in Japan.

Was already in development for PSP, easier to port to Vita as a multi-plat than start development again or something.

Released in January 2013.  Was probably already in development pre-Vita launched.

I'd point out that this game sold well beyond expectations too, which probably led to Experience Inc. committing to the platform for many more games (and Kadokawa too)

Already in development before Vita launch.  Hasn't sold any better on 3DS or Vita so not like there's any alternatives.

In development before Vita launch.  DAN is probably a reaciton to very strong sales of Persona 4 Golden.

Tbh Vita is extreamly good console. It's potential is so much higher than 3DS, but again Sony abandoned their handheld. Sad, because with good marketing and 3rd party producers support Vita could outsell 3DS.
Huge disappointment and I highly doubt that Sony will release another handheld...

Is there some continuation to Death Note I'm not aware of?

Oh, no. That's just how the story ends. Maybe I should have done a spoiler, haha...