Because the bosses were built around the complete lack of knowledge and having guilds figure out as a group what the mechanics were, and how to properly access them over time, which caused many wipes and repair bills via experimentation. Nowadays, the Adventure Guide and PTR stuff spoils everything, which is why the retail bosses fall so quickly compared to back in the days. Also because of class buffs and nerfs that people didn’t math how much this would change for dealing with the fights. Think about how paladins were buffbots initially and now they don’t because of Greater Blessings, which represents a ton of healing added to fights. Also, each tier of raiding is a higher difficulty in itself, which is the same thing with TBC and unlike today where raid difficulty is graded via LFR/normal/heroic/mythic. Also I guess hardware.
Every other reason is people not understanding vanilla.