Only 1 browser to secure! Infosec requires an activeX compatible browser for installing expensive security contraptions. IT can't possibly tolerate Firefox! What, there are more browsers out there? What, Microsoft creates a new browser? What, marketing sent out a new campaign only viewable in Chrome? What, our own site doesn't work in Internet Explorer, and nobody noticed in 2 years?
After fighting their own users for over a decade and looking very sad-kitten-face when every single one of their dastardly back-stabbing users did not follow the holy dictates from above, enterprise IT found out it was actually possible to secure a browser that was not internet explorer. I expect Linux to go the same way. In fact, it did just that for servers.
I also wouldn't characterize it as "harder to secure" but rather "extra work to support." A homogenous endpoint fleet makes all IT tasks significantly easier, and endpoint management tools for Linux tend to be less mature and more difficult to work with.