There are many reasons, including the old chestnut “because leggings are not a suitable replacement for pants,” but let me offer up another: if you are white, pink is a leetle too close to your flesh color. Wearing pink leggings will make it look from a distance like you are kind of naked. And not in a hot way. More in a “why is that woman wearing a shirt and no pants? Who does she think she is, Donald Duck or Wakko from Animaniacs?” kind of way. It will also make any cameltoe you already have look much, much worse.
Also, I am not going to get into the whole “socks with flip-flops” thing right now. Just rest assured that it is not a wise idea.