Firstly, saying one third of kenyans use M-Pesa doesn't really reflect its impact fully - from the Economist's article on mobile money in East Africa last week:

"17m of Kenya’s 19m adults are signed up to M-PESA. Cash equivalent to nearly one-third of Kenya’s GDP washes through the service."

http://www.economist.com/news/finance-and-economics/21579870...

Ultimately, the move toward less regulated currency is driven by the Kenyan Government's decision to tax M-Pesa transactions by 10% which threatens to tip them towards no longer being the most efficient way to move money. This is just a clever work-around to the poor regulation of mobile money.