3rd world currency is often barely worth the paper it’s printed on. Making it crypto removes even that value.
Seriously though, the reason is that they can do digital currency without crypto just fine.
For example there are many popular payment operators in Africa who provide money transfers and micro-payments on phones. M-PESA launched in 2007 and has tens of millions of users:
https://en.wikipedia.org/wiki/M-Pesa
These successes don’t usually get reported because they lack the exciting novelty of crypto, even though they are much more popular and usable. They are actual free market solutions, not coin casinos built around libertarian dogma. And for that reason lots of people don’t want to know.