With handheld gaming not being as big as it once was, it's no surprise more developers don't put as many games on the 3DS/Vita as they once did. The DS/PSP duo had probably some of the biggest software output ever on the handheld market, and meant the highlight of the handheld industry with a whooping +242m consoles sold in total.

It's not weird companies don't put new games on handhelds as frequently as they did. What is odd is that there aren't as many ports of old games to the new systems. The Vita is digitally BC with the PSP, so no problem there, what strikes me as odd is the 3DS situation. Not only it doesn't have a DS digital shop (while being BC with those titles, major opportunity wasted there) but it also didn't get that man PSP ports. Why?

With a couple of exceptions there, I can't really think of any game ported from PSP to 3DS. The games are already made and translated, the cost would be relatively low, so even with low sales they could have made a profit. It strikes me as even more odd with games like the FF, they are on every platform possible but the 3DS, when the Nintendo audience has proven multiple times to support that kind of games.

What is the reason for this lack of PSP ports on the 3DS?

I make this thread because I wanted the PSP Persona games on my 3DS. I'm salty about that :)

Probably because they're held by two different companies. And porting is only possible if the game is not bound by an exclusivity contract.

But there are a lot of 3rd party games on the PSP, and I doubt every single one of them is tied to Sony, specially during the generation that saw 332m non-Sony consoles/handhelds in the mrket.

Can't be much different to the same reason the Wii didn't get PS2 game ports either. It costs money and effort and with dubious chances of getting a considerate return of investment.

PSP games that didn't sell much need no explanation for having no 3DS ports.

There is also Global Warming to take into account.

I think Square Enix is more interested in smart device ports.

But I find it odd 3DS and Vita doesn't share more games.

I mean, my counter question is - why would there be?  I don't mean to sound rude with this, but the argument in your OP doesn't amount to much more than "because I'd like there to be".

You said yourself that the handheld market is shrinking, so why would companies be interested in porting games which are undoubtedly gonna sell less than new stuff?  If we accept the notion that western third-party developers are done with handhelds for a variety of reasons, that just leaves Japan - where the PSP achieved a sizeable 20m install base and had plenty of games selling plenty of copies.  I'm not sure what there is to gain from re-tooling a game for 3DS. 

For examples of ports that did happen, see things like Digimon (220k -> 90k); Kirino no Corda 3 (90k -> 10k); One Piece: Romance Dawn (320k -> 80k).  None of them had particularly bad sales, but not really anything worth chasing either I don't think (especially for the companies involved - Bandai Namco & Koei Tecmo are both mid-sized developers).

The western market has given up on handhelds and the Japanese games that sold "Ok" haven't really sold enough to justify a rerelease on a different system with a different demographic and they don't want to spend money they probably don't on marketing.

If tactics ogre let us cling together came to the 3ds id buy it in a heartbeat.

I would like to see more PSP games ported too but the reason probably is mobile and sales wouldn't be great

Porting to mobile is usually a better option for devs.

Heck, I think there are a decent amount of these games ported on Steam/PC as well.