To quote Ogden Stonehealer during the personal storystep "Assault the Hill" (norn lvl 28 joining the vigil):
"I'd heard the dwarves were extinct. Are there many left?"
Ogden: As a race of flesh and blood, we became extinct over 200 years ago. We made a great sacrifice to defeat the destroyers, servants of the Elder Dragon Primordus.
->"I'd like to hear more about that."
Ogden: Our king protected us with a ritual. Unfortunately, it turned us to stone, immortal but not invulerable (in-game typo). It strengthened our spirits, and the few survivors fight far below the ground.
There's other instances where Ogden re-assures that the dwarves are still existing, fighting, and that he is the sole surface dwarf - who remained to "tell the tale" as he said. But the interesting notion is that the ritual did two things, by all indications from this line: Protected them from corruption and made them unaging, at the price of turning to stone and thus unable to continue "living" as normal beings. They've become the perfect weapons against Primordus - a case of stone versus fire and stone, and the former cannot be corrupted (I suspect forgotten magic had a hand in the Rite of the Great Dwarf's origin). But nonetheless, Ogden does say on numerous occasions outside Arah - of the four times we can meet him, at least three he reassures that there are still dwarves in the depths.
As for magical artifacts - the asura left in a hurry, and to quote the asura lore blog post (transcribed on wiki here
- note, this holds the most info on pre-surfacing asura life that we know):
"Prior to the rise of the destroyers, the asura were the predominant race in the Depths of Tyria. They lorded their status over all others"
"According to the records of the eldest archivists of their race, there were at least six cities as large and grand as Rata Sum in the asura-dominated lands beneath the surface, though none survived into the modern age. While other races insist that the legends of such massive capitals as Quora Sum
are far-fetched and exaggerated, the asura tersely reply that the other races are simply too dim to comprehend the grandeur they lost."
"Inquest founders looked upon the amount of knowledge lost when Quora Sum was wiped out by the destroyers, and judged such a signal drop to be complete anathema to their purposes."
We know the following facts about asura life underground:
1) They were the most superior race, primarily thanks to their magic.
2) They had at least six cities, these six being considered grand. These were all on par to modern GW2 Rata Sum, which thrives
on magic - I mean, that city is a bloody buffet for an Elder Dragon, now imagine one Elder Dragon having free reign to six
Rata Sums. I wouldn't doubt that the Central Transfer Chamber was one such city, given it being an asura gate hub (this was the only settlement to our knowledge near Primordus, since the gates were powered off of him - though to our knowledge nothing else was).
3) On top of that, At least one of these six cities was completely annihilated, meaning little if any magical artifacts would have been recovered.
4) The asura were just as egocentric and paranoid then, if not moreso, than they are now. I don't recall if it was from an interview or from Ghosts of Ascalon, but it was stated that many asura created ciphers out of this paranoia so that their inventions cannot be stolen, thus even after their death many inventions remain lost. This is why Blimm's Golem Eye, or Oola's, Calx's, and Gadd's labs in Metrica Province are never explored and artifacts recovered prior to us helping out/finding them. Now imagine this having gone on for centuries - if not longer - before Primordus' rise.
5) From an interview, we know that the asura already had magic during 1 BE, and they knew it increased and then decreased, though they didn't know why. This means based off of point 4, the asura were likely magically adept for over a thousand years.
We know there were six cities of Rata Sum level fallen to Primordus, and we have no clue how many lesser cities there were, let alone abandoned labs that Primordus can just eat the magical enchantments protecting them right off or send in potentially limitless minions to go through traps and fetch the magic for him.
So overall, I think the reason Primordus hasn't surfaced yet is twofold: 200 years isn't enough to devour all of the asuran magitech (Zhaitan was still searching the outskirts of Orr after 100 years, and the Depths of Tyria is much
bigger), and he's being abated by hundreds of unaging, resilient, uncorruptable stone dwarves that are hell-bent on chasing destroyers down to extinction.