2 reasons for me:
1. compared to all other pools I tried, that's the one earning me the most, even considering the 3% fee
2. there's no correct PPLNS alternative IMO. There's an okay PPLNS pool, but I find its N too small: by the time I used to mine there, I could only mine during the day, and the small N meant that even though it was PPLNS I still wasn't paid if a block was found during the night. Plus it went down a couple of times. So, too much variability (paradoxically, much more variability than with deepbit).
The only pool I'd consider trying now would be a stable enough (uptime) PPLNS pool with N=at least 24h worth of shares.

Additionnally:
Slush has higher fee if you factor invalid blocks, and I really hate that score-based system that was terrible when I was mining with a very unpowerful GPU.
BTC Guild has a history of suspiciously terrible luck. Okay, now that's outdated since they enforce PPS. My new reason for not going there is thus: I just don't like PPS, pool owner takes away the "risk" of bad (and good!) luck at the cost of a huge fee: I prefer keeping the luck for a much lower fee.