Photoshop is full of bloated misfeatures that get in the way of your workflow. The GIMP, instead, was made with artists in mind, providing them with a toolset that makes sense, instead of being done by suits running focus groups.
Photoshop is not the ``standard'' because it is good. It's a classic case of ``worse is better''
[1]: because Photoshop is clunky and hard to use, when faced with a superior tool they can't grok
[2], graphic designers prefer to pretend that the GIMP is not good enough for them, instead of realizing that it is their fault, and the fault of their faulty education. No one has ever been fired for sending work as a PSD file: design schools have absolutely no inclination to stop encouraging Adobe's monopoly. Those schools are not even evil - just completely ignorant of the current software landscape.
References
1:
http://www.jwz.org/doc/worse-is-better.html
2:
http://gimp-savvy.com/BOOK