Bad relievers and terrible, incompetent closers who are also defined as being part of the larger pitching staff. Baseball, like football, is situational and a great starting pitcher who has a 2-0 lead going into the 8th inning also 80-95% of the time needs more runs as a cushion to ensure his good day stands or isn't forked up by a inept, mediocre set of relievers/closers.
A good MLB team like Braves, Yankees, Astros not only have decent-to-good starting pitchers, they also have a trio of great relievers and closers who hold on and MAINTAIN those leads their starters worked their arses off to leave them with. Thats why teams like Atlanta, Cleveland, especially Houston Astros, and Yankees make the playoffs every year and teams like the Reds, Pirates, or even would-be contenders like Mariners don't because they have below average closers/relievers or suck in close games.
A strong set of relievers/closers are one huge, integral difference between why their teams win 88, 89, or 90 wins and why others go 76-86, or finish 4-5 games out of the WC race like Mariners and Twins.