At QuakeCon 2012, John Carmack talked briefly about this decision during his keynote at the event, explaining that Doom 4 requires the services of its mobile branch. Carmack also expressed that he's saddened by the shuttering of id's mobile development -- he's digs tinkering with the platform.
"We did make the decision to close up our mobile development, which saddens me a lot, in that I love doing the mobile work -- taking that time, spending a month, a year or something working on a mobile project, but we had some developers on there that we wanted to bring onto the Doom 4 project," Carmack said during the keynote, as reported by The Verge,
Carmack revealed that Bethesda is looking for grand slams, and apparently so is id. Once it ships Doom 3: BFG, "essentially everybody" at id will focus on Doom 4. A handful of iOS projects, as well as a finished one, have been left in the can.
[via The Verge]