To be honest, Illmatic was an underground record even on the East Coast. I remember Summer of 94 like it was yesterday, Nas wasn't even in the top 5 of people's radar circa 94, in Jersey. The hottest dudes were Redman, Naughty, Queen Latifah, KRS-One, Snoop and Dre, some people were getting into Bone Thugs, too. But in 94, Illmatic was something of a whisper, though some people (myself included) did cop off the strength of the Source review (keep in mind there was no internet to spread the gospel like that). I remembered Nas from the Live at the BBQ joint (tho Ack had the illest verse) and I thought Halftime was dope (came out in 92 I think), but he wasn't really a factor in 94, not initially.
Here is where Nas and people's impressions of Illmatic grew. Around 95-96, when Nas started to do some of his best guest work (Mobb Deep and Raekwon) as well as the rumblings by underground cats that Nas should be a contender for the King of NY when Biggie held that crown somewhat undisputed. Also when It was Written Dropped, a lot of people went and re-explored Illmatic and that's when the praise for the album really started blowing up and then I would say the next couple of years after that, when people were feeling Nas sold out, they then held Illmatic as the gold-standard for his work. So in short, yes it was an instant classic when it dropped, but it was a very slow burn and it took a couple of years for Illmatic to get that sort of near universal acclaim.
That being said, the album is dated, but in a good way, it hearkens back to a grimier NY, where you could still smell the crack on the streets and the gunsmoke from a shootout. Where survival of the fittest ruled supreme, and you had to look out for self.