Because right now, if I put "Fill" as primary or secondary I will get support 95% of the time.
I like variety, I like all the roles, and I liked not knowing what role I'd end up in. Theoretically "fill" should work for me, but in practice it's absolutely awful.
Now I have to decide which two roles I would want to play every game, when I really don't want to.
Note this is coming from someone who plays a ton of support and would even daresay he mains it. But I'm tired of playing it every game.