I've always wondered that.
Many people say so.

http://www.nanacide.com/wahelp/info-better.php

Mainly it's the community, especially the competition / skill level. I first came to wa from wwp because I was told WA had all the legendary players, and I improved a lot by making the switch ;o

Online color maps made me switch from WWP to W:A.

same as bonz, color maps.

WA was always better than WWP in my oppinion. I played WWP only because those I used to play on WA disappeared and those I played on WWP didn't.

The new stuff in the patches has just made it more obvious which is better :p

All the features that make the usage of the game more comfortable. like minimizing, fast frontend and so on.
Just follow Wyv's link :)