Steam makes people buy games. Their discounts and the way it's set up on your PC makes you buy more games than you would if Steam wasn't there. Steam actually increases the money spent.
The other services don't do that to the same extent. I don't know a lot about them, but they don't make the discounts, or anything of the sorts.
That alone makes Valve makes more money (through the cut they get from games selling), but it also puts them in a far better bargaining situation than what the other services have. Put a good, old game on discount on Steam, and it's going to sell. In that sense, I'd assume they can also demand a higher cut of the sales than what the other services can.
I'm not quite sure about this, it's just some basic reasoning. I don't really have any proof to back it up, it just seems logical.
