Because you can only counterfeit the old notes, which have already been inflated to the oblivion: the largest note (1000 shillings) is $0.04. Because the cost of producing a remotely passable counterfeit is also approximately $0.04, the currency is now stable-ish.
If you produce a shiny new 10,000 note, nobody will recognize or accept it.
If you produce a shiny new 10,000 note, nobody will recognize or accept it.