The control key was never intended to be part of the Macintosh interface. Since the original Mac was supposed to be simple and mouse-driven, the only modifier keys were command and option.
Three factors contributed to the inclusion of the control key:
1. The move to ADB. Apple was using the same keyboards for Apple II and Macintosh computers by 1987. The closed-Apple key was replaced by option on the Apple II (note its absence on the IIGS, IIc+, and platinum IIe), the control key was retained since Apple IIs used one, and the command key took on the open-Apple symbol (used very often in Apple II programs, unlike the closed-Apple key). Macs now had a control key by default, and some developers took advantage of that.
2. PC compatibility. Even Apple was pushing this. Remember the PC Drive? Remember early non-Apple efforts such as SoftPC? DOS and Windows computers use control keys all the time. They also have the alt key, which was assigned to the option key on the Mac keyboard.
3. Just a guess, but I have a feeling the Mac team post-Jobs would have wanted to include a control key on the Mac to make it seem like a more powerful computer. (Of course, the PC has always had just two modifier keys--alt and control--although the Apple II series had three from the IIe onward).