Why cordless hammer drill possess such few markets in Europe?What's the difference between hammer drill and impact drill?
I can't tell you why a market does or doesn't adopt a tool, but I can tell you the difference:

A hammer drill has a device in it that provides lateral motion down the tool as if you had a regular drill and somebody was hitting the back of it with a hammer.

An impact driver is like having a that you smack with a hammer in the direction you want it turn.

A good link for this with a few more words: http://www.thewoodwhisperer.com/articles/drills-vs-hammer-drills-vs-impact-drivers/