Why is islam so important to a lot of north african countries?

Well, I am a Muslim, (a person in the religion of Islam), so I could answer this very easily.
Why it's so important to North African countries is because North African Countries are closer to the middle east; the "base" or "home" of Islam.
Why you may ask is it our base/home?
Well that's the area of land our religion was founded in, by our Prophet Muhammad (May peace and blessings be upon him).
So, since North African Countries are very close to the middle east, (they actually used to be part of the middle east; in fact, all countries were actually one full piece of land called "Pangaea" until Earth's tectonic plates divided it into multiple pieces of land), they felt as if they would interpret many of the middle east's customs, religion, tradition, culture, and basically their lifestyle.
This is why it's important :)
I hope I helped you! ^-^

Nigeria and south africa