Hiya oh-great ball-o'energy...
My thoughts on the lack of Organian presence in the 24th Century:
Tired of fighting the Andromedans off so spectacularly that they can't even invade the galaxy. (at least until (if ever) GAW comes out...)
Being imprisioned and killed by those they sought to protect (end of EAW campaign)
Their greatest Prophecy (Fed / Klingon peace) comes to pass
With these three things working against them (along with no "canon" references in TNG), the Organians had unfortunately decided to join the Mirak, Lyrans, ISC, Hydrans, Gorn, et. al. in obscurity...
Either that or you Organians went behind our back, became highly mischeivous, and called yourselves "Q"...
Comeon, oh ye last remaining known Organian. Become assimilated and help the Collective pacify the Galaxy...