@BKrenc Many reasons. A big one is so that the playerbase is centralized; easier for guild recruiting, playing with friends, etc.- Celestalon (@Celestalon) February 1, 2014
BC improved this to a degree. There were still attunements for a while, but guilds could at least rely on the Badge of Justice mechanic and get newer players geared up faster. I actually took some time off playing WoW after Vanilla ended and didn't start raiding in BC until my then-guild was working on the Tier 5 raids (Tempest Keep and Serpentshrine Cavern) and so, in order to get ready to join them, I ran a ton of Karazhan and heroic dungeons and bought tanking gear off of the Justice vendors - I ended up still having to tank in Zul'Aman with a green tanking ring and belt and a bunch of dungeon blues on, but at least I had some gear by that point.
Since those days, we've seen various mechanisms (Wrath and Cataclysm's patch five mans, Mists with justice and honor gear and the Timeless Isle) to allow players to get caught up to current content. And to my mind, while I'm not always on board with the specific way it's implemented, it's the best change imaginable for a variety of reasons.