20 years ago, everyone worked at a desktop workstation hardwired into an office building. This made network security simple and organizations felt they could depend on the time-tested method of the trusted perimeter. Firewalls were relied on to keep out external threats, and anything within the network was considered secure and safe.
Today, however, the number of variables has skyrocketed. The move to the cloud, BYOD, and increased use of outside contractors means a legitimate user could now be logging into the network from anywhere in the world, at any time, and from a vast array of devices.