And?
I'm not saying that its solely the reason for his genius. I'm not saying that MJ is the only musical genius either.
But since this is about MJ it's his cultural impact, dance impact, stage impact, music composition and production, his historical knowledge and knowing where music before him came from, vocal ability, and fashion.
All these things are a part of music for the era in which MJ lived. Just because all of these tools were not available 100 years ago does not make him less of a genius. Every generation adapts in reflection of their environment.