I can't say why no one else plays her, but as someone who used to main her and had her as my most-played champion in seasons 3, I can say why I don't play her any more.
To me, Evelynn was a mage-assassin. Fragile, stealthy, with good burst damage to kill someone she caught out of position, while generally being too weak to directly 1v1 anyone in a fair fight unless she was very fed. She excelled at catching out enemy carries who went out of position, while being relatively weak in direct teamfights.
With the new season 4 vision system, you suddenly couldn't ward against her easily any more, which gave her a burst in power; Riot responded by nerfing her damage output to the point that it crippled AP Eve (changing her E's damage from magical to physical and invalidating her magic penetration / DFG bonus being the final straw.)
Suddenly, AP Eve wasn't really viable any more, and people started to build her as an AD-bruiser.
For me, Evelynn was the stealth-assassin, who had to intelligently use her stealth to outmanoeuvre her enemies and make up for her own fragility.
Now she's just a tanky bruiser that happens to have stealth. The play style has changed completely, and I don't like that new style.
Yes, of course I can still play her as an AP assassin, and do so occasionally, but the nerfs to that playstyle have left her in a bad state, so I feel like I'm not able to contribute enough to my team (and I'm put off by everyone always criticising that build if I play it.)