Me, I learned shooting with counterclockwise rotating elevation knobs so I've stuck to it and all my scopes are CCW. There's less chance of turning the knobs the wrong way in a match since that's the way I've always done it.
It might have something to do with iron sights. Service rifle sights have typically had CW adjustments, which result from using a standard right hand thread with their respective designs.
To get CW adjustments in a typical rifle scope, you would need to use a reverse (left hand) thread.
A lot of the old military scopes have CW adjustments. I think this was due to the design of some of these early scopes, which differed from the typical modern design. My guess for why CW adjustments are offered on modern scopes is that they wanted to keep things consistent with the older scopes and iron sights.