A large portion of those of us who started playing it in beta or the vanilla release, the reason is simple: it became too easy. There was no challenge anymore, no skill involved, just cookie-cutter builds and a lot of button mashing. Some of my fondest memories are of dying over and over trying to kill that one mob. Not an instance boss, but just a plain old elite wandering out in the wilderness. They were plentiful and they were a cast iron bitch. A party of five level 60s taking on Borelgore back in the day when he was a 61 Elite wandering the Eastern Plaguelands had a break-even chance of wiping.
Back then, if you accomplished something, it meant something. You actually had to work for your gear and your levels...walking until level 40, and then getting a mount only if you had saved up the 100g required to buy the riding skill and the mount. Once they started nerfing everything to up their player counts, it began its long slow demise.
I admit, I hung around longer than I should have, lasting all the way through Cata; what a disappointment that was. Pandas? Please. Ihad spent enough of my life (8+ years) doing repetitive dailys. Thus, I'm back to Counter-Strike after a 10 year hiatus.
If you still like WoW, more power to you, but for me, nothing will compare to that first agonizing year, and the joy of finally getting my first toon, an undead warlock, to level 60. What a feeling!