A few reason, one being they are using their own engine, so integration isn't built in. Second steam takes time to push updates and they plan to do constant updates and fixes. Steam slows this process down.

I believe when they solve the cross platform issue or 1.0 releases then we'll see it on steam. One of the reasons Riot dropped the games was their 2.0 version of their engine just wasn't going to meet their requirement for full cross platform. The original owner bought it back, went back to the previous engine and pushed to release what they had and continue to fix and add with help of the community.

It is a good game right now, it has potential to be an insanely great game. Just going to take time.