If all the various flavors of linux are so fantastic, robust and secure (I am not arguing these points), why are they not more prevalent?
The Apple acolytes claim OS X is super, duper easy to use and saves many steps that MS users have to take to accomplish a similar task. The MS devotees claim that MS is the 500 pound gorilla and that the world is MS centric.
I'll provide one answer. The average user can barely operate a computer. Sure they can open a browser, maybe create documents, play with some .jpg files and maybe work with a spread sheet. But really, the average user just wants to turn on the machine and be surfing in seconds.
Apple tries very hard to insulate the user from having to know much of anything about the machine or the software. This certainly has it merits. MS is pretty much the same. Tomato, tomotto.
At the very least, Linux suffers from the perception that it is for nerds or folks that are not afraid to pop the hood and tweak their computer.
As for security, Apple's security is no better than MS if the user logs in as administrator. You think the average user is ignorant of basic file management skills? They are even more clueless when it comes to security. And no one has done an effective job of educating their user base.
I suspect people use the admin account for the same reason they take the guards off their table saws. Convenience. And Linux may do everything and then some, but surely doesn't scream convenience.
Measure twice, cut three times, start over. Repeat as necessary.