I don't really know what the deal is with the low development interest in GIMP. I think maybe it's that Photoshop's professional niche... ...use what they're used to; they're hostile to any change from the start.
I don't know about the developers and why more people aren't interested in helping out (though I suspect for a lot of people its a mixture of the difficulties of getting established in a large codebase and the beurocracy involved in a large project), but from a user point-of-view I think GIMP has a few problems.
I've used GIMP exclusively for many years (but I'm not a graphic designer, so my use of image editing software was never terribly heavy) and I used photoshop for the first time last summer. My transition to photoshop was a very pleasant one as, IMHO, photoshop has a much simpler and more productive interface (buttons are easy to access, convenient keyboard shortcuts, interface is not too cluttered); it has a larger range of (more advanced, generally) filters and tools; and it seems a lot faster to me too (definitely when applying filters to a large image. This can be, in my experience, quite slow in GIMP, but in photoshop most filters are almost instantaneus for me).
I can only assume that many other people feel the same and I imagine this may make it less desirable for people to work on GIMP, especially if there is (perceived) resistance from the GIMP developer community for GIMP to move in the direction that newcomers feel it should (eg, does GIMP still have that horrible multi-window interface? Most people dislike it (though since I started using a tiling window manager on windows it actually becomes much more usable!)).
Having said all that, from a user point of view, unless you make a reaosnable amount of money with your photo-editing, it is still hard to justify photoshops high price tag. GIMP being free is definitely a big plus point for it for casual use.
> "I don't know about the developers and why more people aren't interested in helping out"
This probably doesn't help GIMP any, but I suspect it's because very few people want to hop on such a clearly troubled project. Only two core devs left? A long history of neglect and moving in the wrong direction?
I can bet if, say, Pixelmator went OSS right now, there'd be a sea of devs fighting to add features to it.
Most devs don't want to walk into a major, legacy project just to pick up the pieces, most of us would rather join something that has a solid base where the maintainers have a decent idea of what the product needs.
I don't know about the developers and why more people aren't interested in helping out (though I suspect for a lot of people its a mixture of the difficulties of getting established in a large codebase and the beurocracy involved in a large project), but from a user point-of-view I think GIMP has a few problems.
I've used GIMP exclusively for many years (but I'm not a graphic designer, so my use of image editing software was never terribly heavy) and I used photoshop for the first time last summer. My transition to photoshop was a very pleasant one as, IMHO, photoshop has a much simpler and more productive interface (buttons are easy to access, convenient keyboard shortcuts, interface is not too cluttered); it has a larger range of (more advanced, generally) filters and tools; and it seems a lot faster to me too (definitely when applying filters to a large image. This can be, in my experience, quite slow in GIMP, but in photoshop most filters are almost instantaneus for me).
I can only assume that many other people feel the same and I imagine this may make it less desirable for people to work on GIMP, especially if there is (perceived) resistance from the GIMP developer community for GIMP to move in the direction that newcomers feel it should (eg, does GIMP still have that horrible multi-window interface? Most people dislike it (though since I started using a tiling window manager on windows it actually becomes much more usable!)).
Having said all that, from a user point of view, unless you make a reaosnable amount of money with your photo-editing, it is still hard to justify photoshops high price tag. GIMP being free is definitely a big plus point for it for casual use.