The more jersey designs they have, the more jersey designs they can sell. And pro teams (and some colleges) are all about that merchandising cash. I personally don't much care what the players are wearing as long as I can tell one team from the other, so I don't care how often they change it up.
Now high school & youth teams... that's started to irk me because I don't want to buy uniforms every year. But even with the team providing the standard jerseys, it seems like every season includes at least 2 special games where parents are expected to buy the themed shirt.