Naxxramas was typically considered a guild killer, but why exactly was Naxxramas so hard back in the day? Also, why did so few players in vanilla actually complete it before Burning Crusade happened.
The low completion rate and high difficulty can be attributed by several factors.
1: It was the last raid of the expansion, seeing a lot of people got stuck on previous raids a fewer number of people managed to actually get good enough gear to even attempt the raid. And it was live only 210 days (30 weeks) compared to the other raids that were released earlier.
2: It had an attunement quest that you need to complete before even being able to enter. The quest itself is easy as hell... but it requires a lot of gold or high reputation with The Argent Dawn. So farming rep/gold is needed.
3: It's a 40 man raid, it's already hard to find 10 competent players that can stand each other for several hours... let alone 40 people(something that was an issue for most raids back then). That's why raids were later reduced to 25/10 man and eventually to 20 man.
4: No access to tactics, nowadays weeks before the raid goes live there are extensive guides to how to do the raids with a large amount of add-ons to help the player out (like DBM). Back then players had to do it by trial and error, and based on what they heard from other players they know. And addons were simple and didn't do much to actually help other then basic things.
Also, Naxxramas wasn't the only guild killer. The second boss in Blackwing Lair for example was also one.