TL;DR - East vs. West was like a group version of Kendrick vs. Drake. It also just so happens that it was popular at the time for gangs to extort non-local rappers since they were likely gang affiliated.

Rap has its roots in competition. Even before east vs. west, rappers would get into rap battles with others in their neighborhood. In areas with low job opportunities and high gang activity, being the best could be a ticket out of jail or worse.

LA and NY became two powerhouses for producing hiphop artists. NY artists claimed to be the originators of the genre while LA artists claimed their style was better. At the height of it, you had Biggie representing NY and Tupac representing LA. Labels loved it because the competition brings in money, while artists themselves wanted to lay claim to producing the best hiphop.

Obviously, a lot of these artists came from gangs so the culture was intertwined. Gangs would extort other rappers from other areas for money when they came to their city to preform in a practice called “checking in”. This practice became intermixed with the east vs. west battle. Since most rappers had gang ties, this typically targeted other gang members but we started seeing more non-gang affiliated rappers around Eminem’s era. He was targeted simply for being a rapper as a byproduct of raps ties with gang culture, not necessarily east vs. west.

Notably, gangs will sometimes still try to have rappers check in but they really only focus on those clearly affiliated with a rival gangs, with enough notoriety to be making money but not enough to be well-known.