My longest-running campaign was Shadowrun, actually. Weekly (well, probably 40 sessions a year), for five to six hours a session, for four years. Call it 850 hours?
As for why it went so long ... hmm.
One, I really loved Shadowrun. I love D&D, for instance, but I find myself burned out on it after a fairly lengthy campaign. (My current campaign is edging past three years, and it's basically a race between total burnout and campaign finale right now.)
Two, I found it much more fun in Shadowrun to create adventures and NPCs, and I loved writing them up in mock real world fashion. (For example, I'd write up my PCs' contacts as detailed dossiers for handing out to them, complete with "security cam" photos and the like. No stats, just stuff the PC would know.)
Three, Shadowrun as a system rewards very smart tactics in combat, much more than games like D&D do. In that campaign, my PCs basically figured if a fight wasn't over (in their favor) in two rounds, they'd screwed up and needed to abort. Consequently, character death -- which I've always found to be a campaign jolt (at best) in most systems -- rarely happened in that campaign.
Fourth, the PCs were designed as "competent, but with room to improve" and with the potential for serious emotional development. The leader of the team was a non-cybered, non-Awakened investigator in a very noir vein. By the end of the campaign, he had -- with reluctance and self-loathing well-played by the player -- amped himself with cyber- and bioware so that he could survive and compete. On the other hand, the cybered company woman slid into a descent of alcohol-fueled nihilism which started when she was betrayed by the corporation she'd devoted her life to.
Those PCs, all of them, were by far the best developed PCs in any game I've ever participated in.
Fifth, we were all in college. Life didn't scatter us after a couple of years, which has happened a few times since.