well, the same as the others really: Time.
I think once SMS and phone apps became the norm over having Messenger apps on our Desktops all the time, that was pretty much it for these applications over all. It was a long, slow death. But MSN was one of the firsts to call it quits if I recall right. Oddly the IM app I liked the most. It's just not many of my friends used it. They were all AIM/AOL users.
I always thought WoW was Cartoonish looking. I remember back on Everquest my guild and I were talking about WOW when it launched and we laughed. Called it' "Baby's first MMO". Now mind you Everquest wasn't any better in looks. There aren't many Realistic MMOs out there.