The story
The world
The Art style
The Gameplay (character animations, classes)
The collections
The raiding
Dungeons
My long-term Characters that carry so many memories
Now you'll need to ask the rest of the millions of people still playing and keeping the game alive.
WoW's "niche" is what it's always been? It's an MMORPG with a cartoony art-style. It has more things to do as a non-raider these days than it did pre-MoP to boot, with a setback during WoD.
There are other MMORPGs, and surely there are people playing those that consider them the only viable choice for as many reasons as there are individual players.