Because having a standard is unnecesary and doesn't help anyone. It's pointless regulation.
A self-adopted standard is different though.
Plus Micro USB sucks, miniUSB is more common and better.
I find Mini-USB to be too big these days, since phones are not 20mm thick anymore. Micro-USB is rated to last 10 000 plug-unplug cylcles when Mini-USB 5 000.
But even now, Phone companies can still add their proprietary charging ports as an extra.
My Nokia 6720c supports Micro-USB charging but i just keep using the quick charger.
This is good news if you like to travel.