Because Private Servers are more than likely not filled with bungholes?

Because the possibility of idiots drastically reduces.

You're less likely to experience mic spamming, tking, and kill hogs who refuse to do the actual objectives to make the team win.

it's more fun...basically.

less grind, you get what you want almost instantly and some have a pretty decent community.

Some people prefer having less players online to get a better community experience.