Personally, all of my active
poker sites are ALL connected to cardschat in some way such as for cardschat League games or cardschat freerolls.
Otherwise, I wouldn't see any reason for a hobbyist to have many sites they play at - probably just stick with the site they like most for whatever reason.
The more competitive poker player out there for money (excluding cardschat events) probably would join various sites for some games better at some sites. Maybe one site has softer cash games, whereas another has better MTTs. These are just examples, but the list of reasons is limited only by imagination: different rake, softer games, poker friends, site table graphics, loyalty to a site and so on.
Lastly, there are also government regulations as not everyone everywhere has legal access to all sites. Obviously this plays a role in what sites members go to as one site might be illegal in one country, but a different site somewhere else and so on...