When baseball added four teams in 1969 that made for 12 team leagues. It’s pretty hard to sell baseball when you’ve got a 10th place team taking on an 8th place team.
At first the AL decided to switch to two 6-team divisions. The NL was hesitant at first. However, the NL eventually went along and adopted the same format.
The three division winner + wild card format was adopted in 1994, although there were no playoffs that year.
Since the Cubs last won the National League in 1945, they had made three postseason appearances: 1984, when they lost to the Padres 3-2 in the NLCS, 1989, when they lost to the Giants 4-1 in the NLCS, and in 1998, when they lost to the Braves 3-0 in the Division Series (aka the first round of the playoffs).
The Cubs did win a playoff tiebreaker in 1998 to make the playoffs, but tiebreakers are considered part of the regular season, not the postseason.