EA did not want to use Steam. Thus it's only available on EA's own service, Origin. I think there was a bit of a row about Steams pricing model, and the fact that EA wanted more control over the pricing.
"What do you think this is? Some kind of joke? Gimme' ten Big Macs and a small Diet Coke" - MC 900 FT Jesus
Indeed, though I wonder if having a killer-app game exclusive to their download shop was another factor. They did already manage to get around DLC issues for ME2 after all. Then again Notch said dlc and update policy issues were what stopped him putting minecraft on Steam deespite loving the platform, so it's not like there aren't some issues with it.
I was amused by the quotes saying with Origin they wanted to do something new when Steam still blows it out of the water in every way. You'd have to try pretty hard to out-innovate Valve.
DF: So why can a Marauder roll but a multiplayer Turian can't? Delta1-9: You are fighting female turian husks. You have reach but they have flexibility.