Quite a few reasons actually.
Firstly, because the cheat plugins themselves are not exactly bug free yet. Rincheat, Memdump, Gohanmem, Gohan, are all still works in progress and still require tweaking.
Secondly, I think it has to do with the way values are stored on the Vita games themselves. While certain games store values at fixed addresses, my guess is that several games have either floating, etc values that are harder to find, harder to hack. Example--Demon Gaze--tried using all 4 editors and failed to find values.
Thirdly, cheating is not a priority, it would seem, for the community. From what I have gathered so far,the priority seems to be PSP compatibility, Game Backups, Emulators, Homebrew and then save editing Vita games.
Lastly, the scene itself is quite small. While interest in the Vita has indeed picked up since Henkaku, the number of people doing things is actually still relatively small. The more famous ones are Flow, yifan, Rin, Slade, etc. If you just take a quick glance at the topics across both gbatemp and reddit, while topics have indeed exploded, at least 80% are 'I just got a Vita, how to do X?' or 'Is it possible to get Henkaku from 3.6xxx firmware' or 'Help, how do I do this?' type topics. The people who are actually experienced in hacking, programming and hex editing is still relatively quite small, at least for example compared to the 3ds.