I'll dare to answer this.
Micro USB B is mainly for new devices and its biggest strong points are durability and size. I may be wrong, but I haven't heard anything about using micro ports on computers.
Another point is momentum. There's a whole industry behind mini USB port. Some engineers may not see any improvement with use of micro USB ports if there is enough space on the device. On the other hand, you'll need to redesign devices chassis to fit micro USB port there.
The switch also makes other complications. For example in my country there was (and I think still is) a shortage of micro USB B to USB A cables. And many device makers try to save some money by making the cable an extra equipment (Nokia, Samsung, I'm looking at you!). It is plausible that engineers are aware of such situations and want to skip the problem.
Some people think that there is no need for them to be "first" to push something through. They'd rather wait for micro plugs to completely replace mini before switching to them.
Another point are stockpiles. Someone needs to use up all those connectors. Think about what would happen if everyone suddenly said: I'm gonna use micro connectors for everything! There would by overstocks of mini connectors and shortages of micro connectors. It would take some time for situation to stabilize.
Another thing are people. For example my grandmother has difficulties connecting micro USB B cables, but can connect full size without any problem.
Also, space. Sometimes it just isn't appropriate to use a tiny connector. There is almost no good reason for printers to switch to micro USB B port, well except for greedy manufacturers who'd try to sell cables for just $29.99.
Another point is environment. Take USB chargers for example. Micro USB devices would need chargers with appropriate connectors, so you'd need converter for older charger.
I'm guessing here, but I'd say that equipment needed to manufacture micro cables is more expensive than equipment needed to manufacture full size or mini cables.