Because it works only in small companies and only in theory.
In reality, you'll still get calls to help with $users_favourite_browser and, when refused, calls from higher-ups telling you to drop whatever you doing and go help the user, just this one time, really, because we'll lose a sale like this and we're paying you to do this, don't we and ... And of course you can reject them and point to the policies, but by then you've lost time, gained stress and still may have to go and do it after their higher-ups talk to your higher-ups.
End result - you're supporting single browser on the paper, but in fact you're supporting a browser zoo. No, thanks.
This spoken from a brief experience with a firm that tried to adopt this policy few years ago. Didn't go well. Left them halfway in the process of consolidating the zoo back to "good old" IE 7.