I think there are several things at play:
1. African emeralds have more iron (in some cases, there is virtually no chromium) - which gives a cooler, bluish green (which FWIW I like a lot).
2. Iron also means that less chromium is needed to get the same intensity of green, and chromium is a major cause of reticular defects in stones (emerald and ruby notably), meaning cracks, feathers and all sorts of other stuff affecting clarity are (far) more frequent in Colombian material.
3. African emeralds tend to come in fine clarity and colour, but smaller sizes. This also helps with clarity.
4. Colombian mines are so much older than the African finds (which only go back to the early 20th century), that a lot of the best Colombian material has long since gone. I wasn't kidding when I said above that the good ones went early.
5. Snob value and conservatism: "the finest rubies come from Burma, the finest Emeralds from Colombia". Historically true, but possibly not an absolute truth. Also, as seams get exhausted, perceived rarity (and snob value) is enhanced, just like inherited furniture or silver is considered in some quarters to be inherently more desirable than items bought later.