because it promotes shitty photography. All it does, basically, is give you a handful of terrible filters to slap onto pictures, which are then cropped into a square, then has you load it up with hashtags, then vomits it out into your social media of choice, along with the trademark "taken with instagram" added to the end, as if anyone cares.
However, I think what's most annoying is that since it has all these filters, people think they have to put a filter on every single picture they take. Newsflash: not every single picture needs to have its colors supersaturated or turned black and white. Besides, it's meant to be used with an iPhone, and the cameras on there, while are fairly decent, still give a large amount of grain in pictures that aren't taken in a very bright setting. Also, the people that use it take pictures of the most mundane things, like cups of coffee, their shoes, the sky, trees, because they think it's "artsy." So basically you have crappy quality pictures of boring subjects with garish filters added on top, added with ugly hashtags, all coupled with an annoying ad on each post.