I think the trick is that the micro is thinner, which is good as devices get thinner too.
The other thing is that the micro USB ("μSB" anyone?) is that it incorporates into the standard that it be the actual charging point as well as the data xfer point. Yeah, the USB and mini USB
can do that, but it's not officially part of the standard.
At a guess, I'd expect to see USB/mini to μSB adapters before too long ... whether such would be of any actual use or not.