It's because most of the time, people that use anything but MS Paint, end up using the filters and special fancy tools that come with the bigger programs. These tools are NOT made for 2D pixel art. Gradients do not pass as shading, and airbrushing doesn't either. Not when you have small pixelated objects.
http://forums.graal2001.com/forums/a...&postid=808592
There is a good example of why using some kind of photo editor is not good for pixelated artwork. He used some cheap gradient tool to make a "shadow" which looks nothing like a shadow. The thing is supposed to be a pole, and it looks as flat as construction paper.
Then there are the people that uses lens flares and even go as far as using some built in effects to texture EVERYTHING they make, so it looks like crap, but they think it looks good because they are using some big program with all sorts of powerful tools.
Sure, you can make the same stuff in photoshop/paintshop as you can in mspaint, but generally, people don't just use the line and pencil tools to draw in a photo editor. I just don't understand the people that think ms paint sucks. Like the person I heard once that said "it has limited colors". I mean wtf, do you even look at the program for more than 2 minutes? If he did, he'd figure out he can pick any color he wants, and not just the limited colors given on the toolbar.