CTRL next to A was, I believe, its original location. That's where it was on the Teletype Model 33, which dates back to 1963, and I'm not aware of an earlier keyboard that had a CTRL key.

In the 1970s, most terminals continued with this placement. I believe the modern practice of putting Caps Lock there came in with the first IBM PC -- copied, of course, from their typewriter keyboards.

Being a longtime Lisp Machine user, I had that key mapped to Backspace on my Unix workstations into the late 1990s, but I couldn't do that remapping on my PowerBook, so I gave up -- it was too confusing to have Backspace in different places on different machines. (I think the remapping is actually possible with OS X, but I've never bothered. It's certainly possible with Linux.)