You got it. Check the top 100 selling software apps at Amazon.com, not one Linux application is listed.
And while your at it check what operating system Amazon is running on...
#42 Suse Professional
That's because there is probably a free equivalent that works much better.
Adobe photoshop - the GIMP
Word - Latex
Excel - gnumeric
MSN Messenger - Gaim
Outlook Express - Pine, Kmail, ...
I only chose the above since they are the more popular Windows applications. Most of the above applications are available for Windows, but work more efficiently under a Linux distro. Why pay when it's free?