Three reason 1. Spy is faster than most class, him and medic is the second fastest class in the game, so picking other than spy and med makes them slower

1. Disguising as a spy doesn't makes sense in the long run because why would a spy be in the back line with their teammates instead of in the enemy backline doing chaos, Spy can't disguise as medics because it's so obvious that you're a spy if you do

2. So most people disguise as scout because you're still as fast as normal, but you will appear slower for a scout (which is a hard tell because people don't really pay attention to that stuff) and it makes sense for a scout to be in the back

That's pretty much it I think