I'm not sure that's a big difference. I doubt investors were very keen to hold Greek drachma debt in a similar way that they aren't keen on weak African currencies.
They were borderline... in that Drachma denominated bonds generally carried slightly higher interest rates, but in practice a good portion of national debt was in Drachma. Salaries were certainly Drachma denominated. It may not have been USD, but it was a lot stronger than many/most African currencies.
Besides all the complex economic reasons, Africa just has a lot of very small countries/economies. That makes it hard for them to operate in local currencies and most debt (also salaries, real estate) effectively or explicitly get denominated in foreign currencies anyway. A European equivalent might be Montenegro of Kosovo, but there are 20-30 African countries in that position. Some are reasonably wealthy/developed, but small. Some are very low GDP per capita. Some are both.
