My own personal and humble opinion would be that the mBTC or smaller hasn't been more adopted due to the volatility that Bitcoin still has. Once it is more stable I'm sure that the mBTC or smaller would be adopted more in line with what the dollar and/or euro would be equal to, or any other currency denomination. The fluctuation of where the dollar is at compared to BTC could change drastically in a few hours and keeping it in the BTC standard is the easiest way to keep track of it. But that's how I see things.