Photoshop is chock full of tools and features that a pixel artist wouldn't care about, and they can get in the way. On top of that pixel editors tend to have features only pixel artists would care about: mirrored output (pick one or two planes on the canvas and your work will be mirrored around them), ability to define simple, finite palettes and quickly move between the 5 or so colors you're using often. Also the ability to tweak colors in your palette and have the image adjust automatically (ie replace all of a certain color with another one), but not force you into an indexed color palette. Ability to zoom in very close but still maintain a 100% view, etc.
Most also offer nice animation tools such as looping your animation in real time allowing for effective tweaking. Even other simple niceties like they are always hard edged, always scale with a nearest neighbor algorithm, having both the right and left mouse button be full fledged tools, etc.
The canonical tool was Deluxe Paint on the Amiga, and most tools still around today emulate it at least to some degree.
I used to use Paint Shop Pro for (among other things) pixel art. Somewhere around version 6 JASC decided it was no longer a generic graphics package but should be a Photoshop-rivalling photograph editor. From then on, selections no longer had crisp edges along a pixel boundary but took the edge pixel at half alpha and the next pixel over even more transparent. I couldn't find an option to turn this off anywhere. Goodbye, ability to do pixel art in PSP.
I don't think Photoshop does this (I haven't used it for several versions and my art days are more or less over) but the above has made me forever slightly wary of attempting pixel art in a package primarily sold for photo editing.
Plus, Photoshop is expensive. If you have it anyway and it works for you, that's cool. If you're looking to install something just to make pixel art, you're spending a lot of money on a lot of features which are irrelevant to you and may make it harder to find the functions you actually need.
I still use PSP 9 for bitmapped/raster graphics (other than photos), and had used all of the versions from 3 on except 8. All you needed to do was to uncheck the "antialias" option box (and make sure that the feather value was at zero) to get a crisp (no alpha) selection. The setting were persistent, like a preference, so once you deselected the "antialias", it became the default until you selected it again.
