Because it was always wrong by a strict interpretation of the hyphenation rules of English. In English, one can say "I have anal-retentive tendencies" (with a hyphen) but also "I am anal retentive" (without). Both of these phrases are correct because when two or more words are used to modify a noun that follows, they are hyphenated, otherwise they are not. So phrases like "the control-c character" or "the control-v sequence" are correct, but it should be "hit control c to abort" (without a hyphen) by this rule. However, that's not the full story. When you are telling a user to hit "control c" in a technical manual, when you hyphenate it carries a connotation to many readers (mostly non-technical ones) to press the control key, release it and then do the same with 'c', which as we all know won't work. "Conrol+c" however connotes to many doing both at the same time, so that convention was adopted to avoid the confusion about what '-' means and dodge the rather tricky hyphenation rules (which I've stated only in brief, btw). So this convention shifted as computers became more mainstream.

Warner

P.S. Yes, I know that firetruck used to be fire-truck and it was always hyphenated for a time, even when not used in a phrase like 'fire-truck company'. That's one of the exceptions, and Control-C also fell under that convention. But technical writers started to evolve it to '+' in maybe the 90s to help convey the notion of both at the same time.... and coincidentally to avoid silly arguments about convention vs "the rules" that made things more confusing, not less.

P.P.S. I don't have a good source to this other than second-hand recollection of my wife who used to do technical writing in the 90s, and half-remembered usenet flame wars. The English rule, though, can be found in any style manual, and is direct from a former English professor (also my wife).