Steam takes a 30% cut of the profit last I read. EA tried to avoid this with Orgin to not pay that 30%. I assume Steam sales have to be pretty good VS Orgin numbers keep using Steam.
People hate using extra launchers, and EA has a reputation of being comic book villain evil. I assume any tiny bits of good will they get from customers is rare and this is low hanging fruit. People also love Steam to the point of not buying a game without it. The 30% cut probably seemed worth the trade for the wriggling masses running EA.