It is a program targeted specifically at digital painting. Yes, it can do some "basic" image manipulation like Photoshop does, and even you can do some animation stuff too - but it doesn't intend to target everything image manipulation wise as Photoshop does. I for one as a graphic designer use more tools alongside it, as GraphicsMagick or Digikam's Showfoto, because there's some stuff that it's easier to do with them, or they can do some stuff Krita can't.
People hears that Krita is "a photoshop alternative" in countless comments, blog posts and etcetera and can get frustrated when they find something is lacking. And they can be one of those people that bitch about FOSS because they couldn't manage to do that specific task they can with propietary software.