Because a majority of people dislike playing vs heavies and spies and engineers. The point of the mod is to practice and improve, and there's no real benefit to playing vs those classes. Of course, if you want to, it's totally up to you to edit mgemod_spawns.cfg on your own server to allow any class. (also what hooli said)
as for the question, like hooli said the class limit on arenas is set up in mgemod_spawns.cfg. Just find the map you want to play on, then the arena and then add the class name in the "classes" key-value.
If you need any more help allowing classes in arenas, feel free to add me on steam: http://steamcommunity.com/id/Cprice/