Why is the base game required to enter for a soundtrack?

Nope, because Steam has it as a DLC for the game.

You cant activate it unless you own the base game

Strangely, there are some weird exceptions (At least on steam). I have a copy of the Trine 2 Collectors edition upgrade, which as far as I can tell doesn't actually require the base game to use. When I went to see if any of my friends needed the upgrade it showed every single person on my friend list as being able to activate it. I've been really confused about it, since it's... the UPGRADE. Was starting to wonder if it counted as the collectors edition itself, but it certainly can't.