It's a law. The justification is that it's made by democratically elected representatives. It's also not that much of a problem since we have a 5% barrier. Smaller parties usually have less money to spend on TV ads, anyway.

Note that this didn't prevent new parties to become popular. In the last 25 years or so, Germany has seen the rise of the Green party, and the Left party. Today, the Pirate Party seem to be on the rise. It seems to be a rather stable yet permeable system.