If Linux became the most used OS on the planet, wouldn't more people be interested in finding vulnerabilities to exploit in it? People used to claim Mac didn't get viruses. But it wasn't because it was impervious; it was because it wasn't widespread enough to be worthwhile to make anything for it.
Linux is not the most used but it is used in some of the most important places. An exploit on Linux is still very useful, while there are very few Apple servers still running.