This is part of the book "Introduction to Networking" at www.net-intro.com
In the early days of the Internet, networks were small and all of the routers were in secure locations. As long as each computer connected to the Internet protected itself from unwanted incoming connections, it was felt that there was no need to protect data from prying eyes while it was crossing the network.
So the Link, Internetwork, and Transport layers were focused on the efficient movement of data and solving the problems of a large-scale shared distributed network without worrying about the privacy of that data.
But as the use of the Internet grew rapidly in the late 1980s and literally exploded when the Web became mainstream in 1994, security and privacy of network traffic became very important problems to solve. When we began using the Internet to conduct commerce and credit cards and bank account numbers were being routinely sent across the network, securing data became essential. And when we started using wireless technologies like WiFi, security became necessary for even the simplest uses of the Internet.