Your experience is quite odd. I play with quite a few people that "main" fill, and basically all of our games are all support. I'd say like 70-80% of my games as fill are support, and I have quite a few friends that'll say the same.
In fact, I'm pretty certain most of the community would say the same. Fill puts you into whatever role comes up first, and since support is BY FAR the least played role, you end up getting support more than anything. It just makes sense. And honestly, its working as intended. I have no complaints about the current system.
I just wish there was a way that I could truly randomly pick a role, instead of picking whatever's needed. I want to be able to press "random" and in 50 games, get -roughly- 10 of each role.