Why did only 1% of people see old Naxx in vanilla? Is it that 99% of players were directly barred from entering? No.
There was an attunement, and that attunement could be either a time sink or a gold sink. I'd done it both ways. Neither was enough to explain 99% rejection.
Was it too hard? Hm... Well let's see, we could look at the new Naxx, which is in many ways the same, and see how that went. The 99% appears to be flipped, without the fights being totally gutted. But... gear requirements.
Aha, here we go now. What time does it take to get the needed frost resistance and general gear levels for that raid? That's a tricky question, due to being a long time ago and no longer accessible. I may or may not be still mad about this. Flip a coin.
It's the linear progression that did it. It is theoretically possible that Naxx could have been a walk in the park: zone in with the right gear level and Kel'Thuzad falls over dead and Atiesh is placed in the bags of one random caster, and under these conditions it would still only be completed by 1% of players.
It is Ahn'Qiraj that mattered. And just how many people saw that? I have no clue, because it was more than 1%, and so not as fun of a number to throw around when complaining about inaccessible raids. But how many were in AQ? I already asked that. Why less than 100%?
I blame BWL. And why do I blame BWL? Because it is MC's fault! Now why did no one do MC? I blame warriors. Not for any particular reason, but just because I hated warriors back in vanilla, because at least 99% of them were total douchebags.
As the hippies would say: "It's the system, man. And then I'd tell them to get a damn haircut and they'd call me a hypocrite and they would be right.