From what I've seen, read, heard on QI and such, it's a combination of factors.
Historically, chocolate factories weren't as near to dairy farms as you might expect in the USA, hence more prepping of ways to keep milk longer-lasting. Combine in with companies that raised sugar levels, while reducing cocoa levels. Elements of "company recipes" plus cost-saving. After a few generations of people grow up with that taste, people find it weird if the taste changes.
That happend a couple of years back when American companies bought up most of the English chocolate ones, then tried changing the recipes in similar cost-saving ways. Especially when Kraft bought out Cadbury. Huuuuge public backlash due to the really funky tastes that were so different to what people had grown up with - Brought on by their replacing the traditional dairy milk chocolate with a "standard cocoa mix chocolate" for the Cadbury Creme Eggs.