Intra-Adobe pundits (of the non-executive sort) have from time to time given unconvincing excuses as to why it's too hard for them, but we are all still waiting for any substantive reason why Adobe should not test the Linux market.
Having used Photoshop since version 0.x, I well know that the GIMP ain't the same thing. That's not to say there's anything wrong with the GIMP. Sometimes I wonder why there is no Photoshop clone; the underlying algorithms are more basic than many OSS projects.
Re: the interface, while I would not enter the debate about which is superior, it seems that (viz. OpenOffice etc) adoption of an OSS package is greatly aided if the interfaces are closely compatible (but presumably not patent-infringing, eh, Macromedia?)