Thinking about it, it might be something deiberate.
Cadbury's definitely marketed their chocolate drink as "Drinking Chocolate" rather than "Cocoa" and Bournville released theirs as "Cocoa", so it might have been a consicous efffort not to differnetiate between the drinks, but the major manufacturers.
Indeed, there used to be an advert in the UK for Cadbury's which used the lines
"Cup hands, here comes Cadbury's Cup hands, drinking chocolate Cup hands, here comes Cadbury's now"
but that was probably last screened in the 80's or early 90's.