Is there really any scientific facts to back this tho because my mom is light skin and she's Nigerian
I got light skinned cousins myself and my family and I are African, Cameroonian to be exact.
Historically--and I wouldn't be surprised if a lot of what I'm about to say applies to your ethnic group as well--my ethnic group moved around a bit till we settled at a particular location hundreds of hundreds of years ago, or more. Add the fact that we didn't necessarily shut ourselves out from outsiders AND the eventual encounters with European colonization, forcing us to encounter even more outsiders, African and otherwise, and you have what we have today. This is especially true for ethnic groups in Cameroon and Nigeria. Both countries have over 200 ethnic groups, interacting and intermingling with each other (and with foreigners) for quite some time now.
Meanwhile, as far as we know, the Wakandans have been in more or less the same spot since the vibranium meteorite landed in their area hundreds, if not thousands of years ago. They've isolated themselves and have shunned foreigners (Africans AND non-Africans) strongly, until very, very, very recently. It's not until T'Challa's rule in the comics and T'Chaka's rule in the MCU--essentially modern times--that Wakanda begins to ease up on this.
The Wakandans have a far different history than the vast majority of African ethnic groups, especially those from both our countries of origin, who, willingly or not, ended up interacting with various outsiders throughout their history. The Wakandans willingly didn't move around much (if at all), and stayed to themselves. Therefore, it's very realistic for them to not vary in skin tone as much as we do. The rules that apply to us don't apply to them, due to the large historical difference concerning group movement and isolationism.
Now, Wakanda is entirely fictional and Coogler and Marvel may still go a different direction on this topic. But based on what I've mentioned both in this post and in previous posts, I doubt that would happen.