Here is a
comprehensive look on the Libre Arts site at the current state of free
software for creative
artists.
The other reason is that, with a project like GIMP, it’s hard to do
just one thing. The team is constantly bombarded with requests that
are mostly doable once you have a team of 10 to 20 full-time
developers, which is light years away from where GIMP is now. Which
results in a lot of running around between under-the-hood work, UX
fixes, featurettes, better file formats support etc. So you give
everyone a little of what they want but you end up delaying an
actual release because the big stuff still needs to happen.