Well, the answer for that is pretty much in plain sight. In Africa, there is a big lack of banking infrastructures, that's why Mpesa and some others were able to create a market niche. Bitcoin, has made its way through these options because it has a wider application and wider range. It gives a lot more chances for the unbanked to access Banking services, and with this, they are able to leapfrog directly to this new technology without having to pass through the traditional Banking model.
You are very right. The reason for the widespread use of MPESA is due to a big population which has no access to banking services. This is brought about by the fact that most banks are located in urban centers which are far away, and also, the fees associated with banking are quite high. MPESA came as a savior because individuals are able to access MPESA agents next door and do their 'banking'. Transfer of money has become so easy for them.