That's a very recent development, though, and keep in mind that the government behind it controls less than half of the country, and even that control is often nominal and transient (i.e. the local warlord is allied with the central government at that particular point in time). And the shilling kept circulating at relatively stable rates for quite a while even before the bank was revived.
It's not very recent. The shilling hit a low point in 2008 and appreciated by over 3000% from 2012-2014.
According to what I read the "stability" prior to the central bank was a result of an equilibrium between the value of a note and the value of the paper it was printed on. Since there wasn't a central government printing money, it didn't decline further, but tying up $100 in paper to represent $100 seems pretty suboptimal.