Because of her kit and her ability to be melee or ranged at will. Let me try to explain this...
When Nidalee is played AD its because typically her human form isn't that great. you spend most or your days as an adorable kitty cat pouncing on balls of string we call enemy champs, however you still get all of the harass advantage of a ranged champ so she's ideal for top lane. When her AP side is good she's seen mid. why? because her poke is incredible, and she has heals. she's uber safe and a couple good spears and you can take a tower or dive a champ. Rito in all of their wisdom though decided the width of her spear was BS so it was nerfed to the point she didnt have a high enough accuracy rate to warrant playing her mid.
Rito just recently buffed the BAJESUS out of her jg ability by allowing her to get her passive on jg camps. Part of the reason for this i think is that rito actually likes the champ and her kit and seeing her played. nothing wrong with allowing her to jg though. her ganks still rely on landing a very narrow spear to excell.
BTW, i don't personally think she's toxic or cancerous... i think she WAS when her spear had double its current width. throwing it from behind terrain made it pretty freakin hard to dodge.