One answer is the same answer I gave before to a question about societies in sub-Saharan Africa: https://www.reddit.com/r/AskHistorians/comments/i130tm/questions_about_subsaharan_african_kingdoms/fzvjvyy?utm_source=share&utm_medium=web2x&context=3

The short answer is mostly about the amount of archaeological research that has taken place in the entire African continent, compared to research in other famous urban societies like Greece, Rome, Mesopotamia, Mayan sites, etc. There are hundreds of archaeologists today working in Italy alone, compared to a much smaller number that work in the entire continent of Africa today.

Another issue is not just the total number of archaeologists working, but the amount of funding put toward different research topics. In places like Italy, Greece, or Egypt, the state will fund many local projects, train archaeologists, and sponsor excavations and conservation in museums. There ARE actually lots of local, professional archaeologists in many African countries, but the funding for research is almost non-existent, so most African archaeologists rely on foreign researchers with larger grants. This was not always the case-- in the 1960s and 70s, post-independence but pre-structural adjustment, there was a lot more funding for archaeology within certain African countries. There is a whole generation of African archaeologists in places like Ethiopia, Tanzania, and Kenya who were trained in the 1960s and 70s, but who are aging and will likely not be replaced by younger scholars, since funding and jobs simply don't exist anymore, thanks to the gutting of state programs as a result of structural adjustment policies from the IMF and World Bank, which is another topic that I don't want to get into here.

Specifically, with regard to Aksum-- for one, Ethiopia was off limits to most foreigners, including archaeologists, for a very long time. It was the last African nation to experience European imperialism, fighting a war against Italy in the 1890s and winning, and then being briefly occupied by Italy in the 1930s. Through all that, it was never colonized, meaning that European archaeologists were largely never able to excavate and explore they way they could in places like Egypt, or other parts of East Africa, like Kenya or Tanzania. Outside of the atrocities of colonialism, one thing that did absolutely happen was that archaeological research boomed in colonial countries. To this day, the majority of archaeological information we have about Mesopotamia and Egypt comes from excavations that occurred under colonial governance, simply because archaeologists then could pull off massive, HIGHLY UNETHICAL, projects. Archaeologists were a key part of the colonial apparatus, and they were funded and supported accordingly.

Then, during the Cold War, Ethiopia was still largely off-limits, due to the civil war from 1974 to 1991.

So research only really began post-1991, and still, it's quite difficult to do research there, due to the language barrier. If you compare this to say, Italy, which has been picked over by archaeologists for 200+ years, it makes sense why there is so much more known about ancient Rome compared to Aksum. But there is lots of interesting research happening currently though, which I could link to but it's all articles behind paywalls so I'm not going to bother.