Plus Paint.NET occupies a sweet spot: it is simple enough to easily do easy tasks like cropping, while complex enough to do so slightly more advanced things.
I find it to be especially popular among IT professionals as they usually don't need to do photo-manipulation ("photoshopping"), but need to process images every so often.
There are many people who can't afford Photoshop, so Paint.NET is useful for those who want something better than paint without having to sell their car to pay for software.
