It's because he fell off so quickly.
From '88 to '92, he was considered one of the best, if not the best, rapper in Hip-Hop (yes even better than Rakim), but then after The Predator, he let everybody down with Lethal Injection, a good album, but not up to the standard of his first 3 LPs and EP, which clearly was the beginning of the end, although the fall off actually started with The Predator, that's album where he started to get into that rappity rap gangsta shyt and less about race politics, etc.
Cube ran out of things to say, his strength was talking about racial shyt, even tho he, along with his N.W.A. band mates, popularized the gangsta shyt, that's what killed him post-Predator, he stopped talking about the things (race, politics, etc) that made him shine and just became another gangsta rapper, from there people lost interest, and his music worsened, even up to this day he still hasn't recovered.
He surprisingly rebounded with his last great work in '96 with the first Westside Connection album, Bow Down, although it was a return to form, some even found that to be weak because, as I stated above, Cube shined on the mic when he talked about racial politics and stuff of that nature but after Bow Down, he pretty much shyt the bed outside of a few singles here and there.
Cube is in my top 5, but sadly, his prime didn't last very long, East Coast Renaissance kicked off and the West Coast started changing into a new territory, G-Funk/Death Row sound, that he was somewhat ill-equipped to handle, and plus some might say his albums haven't aged well, due to the sound he was going for at the time.