First, Cadbury's was the market leader in the UK (and elsewhere) and was more than holding its own. Hershey didn't even have a presence over here, so it was hardly the case that it couldn't keep up with competitors either here or abroad.
Second, what happened was that Kraft Foods (now Mondelez) launched a hostile takeover after the board of directors and shareholders played the 'profits are down' trick one too many times (in reality, what they always meant by that was 'consumers: prepare for smaller bars at the same, or even a higher, price', because they always managed to attract new investors; that tends not to happen if the continual profit warnings are real
).
Whatever happened to the US side of things, rights, etc after that happened on Kraft's watch, and presumably they thought they were being ever so sneaky. In any case, it was only after the takeover (and whatever deals followed) that we began seeing Hershey's here, certainly as a mainstream product.
In any case, Cadbury's is just a trademark now. The processes have already been altered, operations bunted around or closed down (with production presumably moving Stateside).
The shareholders managed to get a price they were happy enough with in the end, and over here that's all business "leaders" really care about - give 'em the money and they'll gladly run, saying 'Sod you all!' as they go (most of them would sell their mothers for tuppence).