In order of most importance.. 1) Because I'm always looking for something new to play. 2) Because it's F2P. 3) Because it's the Forgotten Realms/D&D universe. I was a fan of NWN1, so I thought I would give NW a go. 4) Because some friends play.
The fastest MMO death that I know of was the original All Points Bulletin. It was released & not even a month later it shut down. Tho it did get resurrected some time later after it's initial demise as All Points Bulletin:Reloaded.