There are a tremendous number of users out there that use their computer primarily for surfing the net, email, and a little word processing.
Ubuntu does all of those thing wonderfully.
It is all I want to have on the net. If I am going to do a lot of processing, I will probably do it off line in a dedicated computer so that I don't have to worry about compromising my security.
You are a part of a small minority of users. Perhaps that group will begin to grow, as web apps become more prevalent and installed apps become less important. But we have a way to go to get there.
Of course, if Linux starts to become more prominent, the amount of malware to be written for it will increase as well...