When, where, and how did rap change?

How come the 20' rap is so much different from the 90's rap? Why is the 90's rap different from the 80's and so on and so forth. How does it all change? Anyone in specific ever change it? Do you, maybe, have charts I can study? Is it true that the deaths of Tupac, Big L, B.I.G, and Big Pun ended the 90's rap? Which period of rap do you like? Why? Why did become West side vs East side? Why does it seem that its starting to become North East Rappers (NY, PA via Meek Mill, J. Cole, Jay Z etc) vs Down South (Lil Wayne, Kirko Bangz, etc) vs West Side (Omar Cruz) vs Central US (Kanye West, Common) I can go on but you get my point.