Nost was the primary reason. In the history of (vanilla) private servers there wasn't a single one that had its amoubt of polish and custom changes to accomodate to a better simulation of the original release schedule of content. The closest before Nost I remember was Emerald Dream (2012) which iirc buffed bosses and released raids in a timely manner akin to the origibal schedule.
Nost did this:
at launch it had pre-1.6(?) itemization
pre-1.4 epic mounts
dire maul wasn't in at launch
debuff slots at 8
buffed bosses (applies to the raids releleased)
higher server cap of 8k at the start (only PvP server esxisted) up to 15k by the end a year later (one PvE and PvP servers each exsted)
iirc relatively few to no drama created by shady dealings of the leadership
true "blizzlike" with a strict x1 exp rate and dynamic respawn
1.4 epic mounts were removed access to by the time BWL rolled around or shortly before iirc
itemization fixed like it was done originally
debuff slots increased to 16 when BWL launched
Naturally, AQ never released on Nost itself because of the shutdown. The irony is that Noszalrius had probably so many custom changes made to it that it wasn't much more or less of a true vanilla experience than other private servers were. What made the difference were the changes made to emulate that original release schedule as closely as possible (talents for example were never adjusted to their oftentimes useless states like it was in OG vanilla) and the amount of polish and of course the player numbers.
Before Nost most servers usually hovered around 500 to 1k pop, 2k on the "big" servers like Emerald Dream or Molten servers. By standards layed out by Nost a lot of people suddenly started calling those servers dead despite still having a relatively healthy population. Heck, I played once on a german vanilla pserver that barely scratched the 500 pop and still managed to have a few guilds competing in MC, BWL and AQ (I think). Yet, those servers were usually riddled with a lot more bugs, custom exp rates and drop chances, potebtially a cash shop fueled by "donor" or "voter" points or both, in which case only do or points usually allowed for the buying of mounts and gear (like it was on Feenix' other servers. Not on ED though). Not only that but those servers usually had very corrupt leadership or GMS that were generating gold to give to gold sellers to get a piece of the profit etc. There's a reason why Nost's successor, Elysium, imploded and had another successor Light's Hope which had similar amounts of distrust as other earlier pservers, but was still carried by the Nostalrius legacy.
One thing that definitely helped generating Blizzard's interest of making Classic wad the absolute failure of WoD. It can't be understated of how much it failed. And no, it wasn't because "haha retail bad" but it mostly came down to shitty management at that time, as we know something happened during its alpha that changed the direction completely by stuffing anything and everything into the four walls of garrisons. Remember the expac was so hyoed that subs increased from 7.5m (last report before the expac) to 10m by launch. Ofc they dropped to 5 million by the end of 2015 after which they stopped reporting the amount of subs. And that's where Nost came in. Just three months after the majority of players left retail because of WoD a new polished quslity Pserver of the "legendary" vanilla version of the game launched. With loss of trust into retail loads of people flocked to it. Never before did so many people flock to a private server. I know thus sub hates them, but when people like Sodapoppin (who wasn't a true variety streamer at that point) start talking about it you know it will gain traction. All that contributed to the success of Nost and generate interest for it with Blizzard.