Because most people play with six Pokemon on their team, and find the threads most helpful if there are six Pokemon on the answers. You can debate the worth of it, but it won't change that reality.
The threads are set up intentionally to fit the average person playing the game. If we start allowing responses with variations that don't follow the norm, the threads become less helpful to people overall.
If you want to post a team with less than six Pokemon, mention a few sets people can use to fill the rest of the slots if they want. There is no need to use these threads to force a gamestyle upon people.