In a nutshell, undecided voters are apathetic:
A large chunk of people decide late because they just don’t care about politics and tune it out for as long as they can. Tulane’s Brian Brox and the University of Arkansas-Little Rock’s Joseph Giammo examined the attributes of late deciders in presidential general elections and found that a large chunk of them resembled the “stereotypical apathetic citizen.” They tended to be less partisan and politically active than early deciders, treated the candidates as interchangeable, and weren’t inclined to care who won.
One surprising twist: Brox and Giammo found that these “low-interest late deciders” seemed to recoil at information sent their way. The more Republican ads they saw, the less likely they were to vote Republican; Democratic ads pushed them toward the GOP. “These voters may actually be likely to become irritated with the efforts of candidates to attract their votes,” Brox and Giammo write, “since we know that they have little interest in the campaign in the first place.”