Either because they suck, were better at older COD games, permabanned or really dont like it.
Plus gamers can't be satisfied. People complained MW2 and act as if Treyarc was a savior but now they hate Black Ops and love MW2 again as well as IW.
MW2 is a better, more fun game imo but Black Ops is way more balanced. No "overpowered" killstreaks, there are overused not truly overpowered guns, Noobtubing is normal, Ghost vs EASY SPY PLANES, 2nd Chance only real issues, no derank lbbies (yet) but people still complain. I like "overpowered killstreaks" and the weapons from MW2 myself.
Some like big maps for sniping and battles, some smaller, less campy maps. Hell some think Hardscoping is a sin. We can't be satisfied. MW2 was unbalanced..,BUT FUN. Black Ops is balanced but sometimes fun. Can't please everybody.
Also most who say it sucks buy DLC and play it 24/7 so they just like talking.