Why is Mountain Lion more security conscious?
Apple computers have long been considered "virus free" although Apple stopped pushing this as a selling point earlier this year in the wake of several Apple targeted viruses.
The truth is that a virus can be programmed for mac much like a software application can, the reason for the lack if infections on mac was merely the same as the reason for lack of some apps in the past, the relative unpopularity vs windows machines meant it was less practical to program for mac.
With macs now being more popular than ever before, Apple now has to implement strict measures to hold on to their clean, virus free image, hence Gatekeeper being added to Mountain Lion.