Says who? The East Coast biased media? The Source magazine that underrated The Chronic and Doggystyle which made them re-rate them as 5 mic classics years later? The East Coast radio stations back in the '90s which refused to play West Coast music?
Breh, at the end of the day, your gonna feel the East Coast has more classics than any other region but you cannot deny the bias
A lot of West Coast albums are just as good or better than East Coast albums. But Hip Hop fanboys who actually know nothing about music, regional cultural significance can't formulate their own opinions about music that hasn't been reviewed by whiteboys from White Plains, New York who worked as music critics for big publications back in the 90's.

Messy Marv & San Quinn's
Explosive Mode is musically better than any album that came out of the East Coast in 98'. 2nd II None's debut album was musically as good as any classic album from 91'. Dru Down
Can U Feel Me was as good as anything back in 96'. But these albums aren't constantly promoted by hipster whiteboys who moved to NYC from the Midwest in 2008 or their older brothers who wrote gushy reviews about 90's NYC albums. That Korean lady Miss Info was the original critic who wrote the 5 mic review for
Illmatic.

Ironically, most of these critically acclaimed East Coast albums sold horribly in the 90's and were criminally slept on despite the fact that 90% of relevant NY rappers were signed to major labels.

I'm 32. By the end of the 90's, people looked at Nas's career like it was a joke.
Illmatic went wood and no one really started talking about it again until Jay-Z dropped the "Takeover" in 2001

,
It Was Written was met with mixed reviews upon it's initial release and
Nastradamus was panned.