It is, in fact, just bad luck. My favorite go-to example is the 2003 Tigers, who lost 119 games and had an awful bullpen but still managed to 19-18 in one-run games.
When a team is experiencing negative variance, it's really tempting to say "ah-hah, the reason is all the things they've done that I've disagreed with" like not calling up some prospect that everyone wants up, or the manager doing whatever managers do that people don't like, or employing Ian Stewart.
While those things certainly don't help, and the Cubs would be better off with a better bullpen, they really did just experience negative variance. So it goes.