There's a lot of factors to this, and you touched on one. It was the formative years of rap, so reigning supreme for 3-5 years doing that era holds a lot more weight than it would in this era. Whenever you're one of the forefather of something and helped lay the foundation of a majority of respected artists, and some these artists are ones that have actually been able to advance the culture even further, came up listening to you as their idol, over the years, and were their influence growing up, that means so much more than say a Lil Wayne who had a hell of a run, but didn't have as big an impact on respected and revered artists. Now, that can also be considered unfair because Wayne's reign maybe hasn't been removed long enough to see the fruits of it be displayed in another artist that turns out to be a legend, so we'll wait and see.
Another reason is because of bias. We all tend to lean towards artists that we grew up listening to in our early years as being superior and also tend to ignore a lot of their faults and missteps. Just as there's kids who are 20 now will tell you Drake is the best rapper ever and think Ice Cube is an actor and laugh at the notion of anyone thinking he's on Drake level, just because of how they were exposed to him originally. They grew up seeing Ice Cube as the comedy movie dad, friend, cop etc...who used to rap back in the day, but isn't really known as a legend in that field to younger kids. They don't respect him in that light, because they're not familiar with his history.
Perspective is such a funny thing.