Because they screwed the small miner over with a wacko payment system and kept all the NMC (back in Oct 2012). I wouldn't mine there again.
It's all on record in the pool thread somewhere around Oct 2012. I was warned not to use a 0% fee pool. But I was newbie and didn't listen.
A pool can extract wealth from a miner in more ways besides a % fee, like using delaying payments for currency arbitrage, or loaning to shorters, or secretly merge mining an alt coin.
A pool or exchange can be crippled if it's caught on the wrong side of an arbitrage bet. This is where I suspect many "we were hacked, the coins are gone" problems start.
"When the tide goes out you get to see who's wearing shorts" - Warren Buffett