The designer has answered your question, here:
https://medium.com/@gregbunbury/where-are-all-the-african-flags-50dde5dd1ea6
The long post is well worth a read if you have time, if not, here's a short quote:
"the topic of imperialism and the legacy of global colonialism is voluminous, to say the least.
"The reality of producing an artwork that can address such a gargantuan concept, while speaking to all experiences and perspectives, felt not only unfeasible but problematic. And to do so would undermine the very reason the project exists, as per the question: βWhat do Black people want?β". 1/2
"Well, Black culture is not monolithic, and one size does not fit all. In addition, I do not believe a large enough billboard exists, to graphically capture the full scope of the topic.
... "This is not to reduce, ignore or sideline African nations or other cultures throughout the world, that have suffered and survived colonial and imperial struggles.". 2/2