IMO I think it is because like 45yr said his lyrical ability and I don't care what anyone says even during the whole east/west beef alot of cats from the west dug his music. He was able (like Pac) to make music anyone into hip hop/rap no matter where they were located could relate to. I think to this day that if Biggie had a chance to make more than 2 albums (I dont really consider the 3 his because Puffy put people on there Biggie never would have worked with) he would maybe the best MC ever.
Anyways RIP to Biggie,Pac,Big L,Rappin Ron,Cougnut,Mr.C,Hitman,Soulja Slim,Mongo and all the fallen mc's that were taken before their time