Beause two teams is a simple, right to the point tdm. Most people don't want to build a map to support 3 or 4 teams when they could just make a fun map with two teams, although I would agree it would be fun to have more teams except for the fact that then there would be more players on other teams and not many on yours, so you would be doing lone wolf stuff most of the time. Another thing is that in sieges and cops vs robbers gamemodes, having 3 teams doesn't work, and not all tdms are actually the tdm gamemodes. Many people like to customize they're tdm to make it special somehow. Such as heedicalkings winter tdm, there was no way a third team could be implemented, it would be stupid.
That's just my opinion.