It's because Poppy was originally created as a laner, back in the day when champion kits weren't overloaded with range and mobility. Try laning with Poppy today , you will get kited and zoned to infinity and be unable to do anything. However, Poppy is quite good as a jungler, it's just that her ganks are difficult to pull off because her CC is situational and her early game damage is not all it should be.
However, her passive makes her insanely tanky, especially against high damage mages and assassins, who will rarely if ever be able to kill you in their full burst. Her passive also synergizes well with resistances, making her unbelievably tanky with just a couple of defense items.
I play Poppy jungle quite a bit, and to be honest she is broken. It's just that she takes time to learn how to play properly, and considering how old she is and how all these new, more appealing champions are being released, most league players don't give her a chance. I do get worried though when I see a Poppy on the other team, a good poppy is a menace mid game, and because of her insane tankiness and invincibility ult, she is one of the few assassins in League who does not fall off late game, which can make her a nightmare to fight against.
Also, few things are as satisfying as targeting a squishy with your ult, walking through the entire enemy team as they all spam their full ability set on you, killing the squishy in two hits, and walking away.