Research is a wonderful thing:
"In 1982, ESPN began showing the opening rounds of the NCAA tournament, which established ESPN's following among college basketball fans and was the network's first contract signed with the NCAA for a major sport. According to many fans of the tournament, ESPN was easily the best broadcaster of the first round, as six first-round games could be seen on both Thursday and Friday on ESPN, and CBS then picked up a seventh game at 11:30 pm ET. This meant 14 of 32 first-round games were televised. ESPN also re-ran games overnight. ESPN did not (and does not) have regional affiliates, so the entire country had to watch the same game; there was also no ESPN2 or other channels. (Areas with local interest in a game could see the game on a local channel, regardless of which game ESPN televised.) The benefit of this was that ESPN always showed the most competitive games, since that was the best way to gain national appeal.
In 1982, CBS obtained broadcast television rights to the tournament.
In 1991, CBS assumed responsibility for covering all games of the NCAA tournament, with the exception of the single Tuesday night "play-in" game. (The play-in game - between teams ranked 64 & 65 - is televised by ESPN, except for the first one, which was aired on TNN, and used CBS graphics and announcers.)
Currently, CBS broadcasts the remaining 63 games of the NCAA tournament proper. Most areas see only eight of 32 first round games, seven second round games, and four regional semifinal games (out of the possible 56 games during these rounds)."
http://en.wikipedia.org/wiki/NCAA_Men%27s_Division_I_Basketball_Championship