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).
