I did so initially because:
1: When I was studying Japanese during uni and watching fansubs back in 2003 onwards, I often shouted and got pissed off at the inaccurate translations and editing errors that I saw.
2: When I graduated from uni, I wanted to retain and build my Japanese skills, I had plenty of time, so I joined a fansubbing group. It changed my life in more ways than one
3: I appreciated the skills of the other guys in the team (timing, encoding, typesetting), enjoyed learning about these skills (me and encoding = never) and seeing the process it takes to make it from raw to release.
4: I still wanna build enough interpretation skills enough to interpret and translate for natives while I'm still in anime-land. Japanese to English is easy, English to Japanese, still needs working on, I hiss at born bilinguals who understand both sides effortlessly...
5: At present, I translate my own personal one off projects (rather than series), translate officially for a Japanese charity as well as deal in performance arts in both languages.
Fansubbing for me was also being part of a wacky, insane community that I think people (geeks) are too shy to engage or expose their true selves as freely offline. (People can easily write wise cracking comments online, but cannot say them as snarky offline, especially when dealing with females irl, I noticed)
Online, I saw the horrors of the imagination of some of my peers in a level of depravity that otherwise get us all arrested "in real life", but I also saw the kindness that if not most guys hid behind their online persona of acting 'tough'.
IRC has been a fascinating experience that few experience in this crazy world (your average streaming anime teen doesn't even contemplate where the subs come from, let alone know what IRC is).
Also we're all getting older, so when work, relationships and family come into the game, one's priorities in life changes.
Fansubbing then takes a backseat if not total closure.
In my case, learning a language, especially Japanese is a "
until death do us part" deal, so I'll be at it for quite some time yet.