I mean, I kinda disagree with that, A lot of artists didn't have that issue. We can take Kendrick as an example... many are now ranking his first major album (gkmc) as his weakest, despite the massive acclaim it got.
Same shyt for Kanye West, Outkast,,...whats their best album? there's no consensus, you see each and every project of them being praised as their #1, just like Kendrick.
I'd say the reason why Illmatic is the unanimous answer when talking about nas best album is because it's by far his most acclaimed project, from critics and fans. It also doesn't help that most Nas albums sound "similar" (same NY style).
On the other hand, artists like Kendrick, Kast, Kanye manage to reinvent themselves with each album, to the point where you could say each album was made for a difference audience and since all of their records get similar level of acclaim as their debut album, it's almost impossible to determine their best record.
When nas makes an album, you already know he gonna make it for the same Nas fans that been riding with him since day 1. Kendrick, Kanye? you don't even know, they might record a 100% funk album that rap fans might not enjoy but it would still be an elite album from a musical standpoint, praised as a masterpiece by the music/funk community. And while Some "rap" fans would argue GKMC/TPAB/DAMN/MBDTF/CD/LR are better, that funk album could still end up being their magnum opus, determined by the rest of the music community.