Why Won't Johnny Run Linux? - Problems with Commercial Applications
Key applications missing

People want Photoshop!

Gimp's not (yet) a sufficient substitute:
Unfamiliar UI
Can't do CMYK images
Can't do 64 bit per pixel images
Can't run third-party Photoshop plugins

Photoshop has a real plugin ecosystem
Other apps (e.g. Paint Shop Pro) accept Photoshop plugins