Because classic WoW was hard. The new expansions have been made extremely easy so everyone, instead of just people who are good, can see all the content. This makes many players mad.
Back in classic WoW, you needed 40 people to raid. There were no "heroics" (five man raids), 10-man raids or 25-man raids. You got 40 people or you didn't have a raid. Even then, the raids were extremely difficult, sometimes taking dozens of tries before downing even a single boss out of several. BC and Wrath can't do that.
BC had all 25-man raids. While easier to do and complete, many players were upset at how easy it was compared to classic. Many of the classic raids required players to take weeks getting the right gear.
Wrath is a total joke in comparison. In the first week of Wrath being released, guilds had cleared ALL raid content. Within two weeks, entire guilds were covered in all of the best gear in the game.
Wrath was dumbed down, and it shows. No longer only people who try win, everyone does, and that's sad.
Back in Vanilla, epics were just that, epic. They were hard to get and not everyone had them. You had to actually do the 40 man raids, or spend weeks in PvP to get them. It wasn't like it is now. Now, you can just get five people together and do a heroic instance in an hour, and get epic gear off the bosses.
PvP isn't helping either. Back when you had ranks, only the best player on your server on each faction got the epic gear. You had to grind all the way up to High Warlord (or Grand Marshal for you alliance players) to get epics. Now, if you just slam your head against your keyboard in a battleground for a whole day, you get epic gear. You can literally get epic gear in BGs after ONE DAY.