Okay, I'm just going to talk out of my rear for a bit... and hypothethingy..
In the 80's (when I was at school) all of the kids had bikes. There were choppers (I never had one and I am emotionally scarred as a result), there were racers, there were shopping bikes (basket on the front), etc. Cycling was not a hobby, it was truly the preferred form of transport for a very large number of people. These people most probably were not part of any cycling fraternity. If they had been then they would probably have come across the term "road bike". The shift between calling it a racer and calling it a road bike, I believe is as a result of the bicycle falling more into the realm of hobbyist (excuse this word, it may not be quite right for the description). The car has become more prominent since the 80's as the preferred way to get around. This means that a larger number of bike buyers are from the cycling fraternity rather than what I would describe as the general public. With the advent of the internet and forums such as this, the whole thing has become so much more condensed.
Unfortunately, how ever much I might hate the term "Road bike" it is now the formal description of what the majority termed as "Racer" in the 80's. When I say "Majority" I do mean majority because there were probably more people riding racers to get from A to B than there were enthusiasts (thats the right word).
Of course the above may just be a load of bull.