Community projects do tend to bear higher completion rates, as long as the community doesn't immitate the metroid prime 2d community. The mp2d project was "founded" by Andrew Mills(I believe), who crowned a few random people moderators and expected the community to create the game themselves. The mp2d project's progress has been horrendous, it was started in 2003 and they have only established an incomplete engine and done the first 5 rooms of the Space Frigate(first section of game). However, the OOT 2d community project here has demonstrated astonishing progress, and if not completed I believe they will at least surpass every other OOT2d fan game.
The conversions themselves usually aren't completed because of several reasons...
1.The game is attempted by one person, who can't possibly complete such an immense project.(Doesn't always apply)
2.The staff of the game, whether one person or a group, is too inexperienced and don't acknowledge the enormity and potential hardships of their undertaking.
3.Loss of interest, usually because of a programming impediment is encountered.