The obvious reason why very few non-enthusiasts use linux is that Microsoft has abused their monopoly powers since the late 80s. Try to find a vendor who will sell you a dual boot system? You cant find one.
Until recently you could not get a Dell or HP with linux preinstalled. And even now you have to know what you are looking for to find the option.
If a PC arrived with a Vista CD an Ubuntu CD and a checkofflist.
Vista - Install, install drivers, reboot, sytem update, reboot, install antivirus, install anti-spyware, install office software.
Ubuntu -install, update wireless/video drivers, system update and reboot.
On that playing field, Linux would hold more than 2% of the desktop market.
Everyone knows someone who knows enough about windows to get going with it. If the same was true for Linux. Microsoft would be in trouble.
The only problem business would really have, is if their applications would run in a linux environment. Believe me, I would rather train an XP user with Office 2003 experience to use Ubuntu with Openoffice before I would want to train them on using Vista with Office 2007. Vista with Office is just to different from what they are used to.