John Carmack: "Way back in March when I released the source for Wolfenstein 3D Classic, I said that Doom Classic would be coming "real soon", and on April 27, I gave a progress report:
I spent a while getting the multiplayer functionality up, and I figured I only had to spend a couple days more to polish things up for release.
However, we were finishing up the big iPhone Doom Resurrection project with Escalation Studios, and we didn't want to have two Doom games released right on top of each other, so I put Doom Classic aside for a while. After Doom Resurrection had its time in the sun, I was prepared to put the rest of the work into Doom Classic, but we ran into another schedule conflict. As I related in my Wolf Classic notes http://www.idsoftware.com/wolfenstein-3d-classic-platinum/wolfdevelopment.htm , Wolfenstein RPG for the iPhone was actually done before Wolfenstein Classic, but EA had decided to sit on it until the release of the big PC / console Wolfenstein game in August."