Because she has a short range, now she has no scapes and nothing to compensate for building Glass Cannon + Full Squishy and with a really low range. Other ADC's can do the traditional ADC job better.
Instead she gets really rewarded for building AD off-tank, and that is why she is played in top, and because she can keep a melee at bay, and this is also the reason why she is good in the jungle, the jungle rewards her for being a semi-tank.
She is really dependent on snowballing now, if she does not get kills with the roam she will loose farm and gold. Also it is easy for a team to notice when a support is alone on botlane (or when botlane is empty). The problem is the same on the midlane and is even worst because you have to fight champions that can outrange you, poke you and burst you down, and you hurt the team composition because you loose the AP carry or the Assassin as said.
In the jungle no one knows when u are roaming, the jungle monsters can't poke you and her ganks are really scare, all her powers are good for ganking (ALL OF THEM -they always were good at it-). Her Passive can allow her to track an enemy on the bushes, the Q is good to burst down an enemy, her W is good to chase and Scape for the passive, her E is so damn good to slow down an enemy and kill it, and her ult is now super good to enter and gank (and roam), even if I miss the assasination (which I prefer, but I will adaptate.... I believe).
Also her whole new focus is to create uneven fights, a gank is an uneven fight, and as a jungler she can create uneven fights quickly, one after the other in a row, it is really good.