My theory is that it's a result of institutionalized corruption: neither party wants to win by a landslide anymore. They want to win, in Dick Cheney's words (quoted in Obama's biography), "fifty percent plus one".
They want to share the spoils of victory with as few as possible. Winning with a big margin, to the party apparatus, is evidence that you wasted valuable political capital on pleasing voters that could instead have been spent on pleasing donors.
They want to share the spoils of victory with as few as possible. Winning with a big margin, to the party apparatus, is evidence that you wasted valuable political capital on pleasing voters that could instead have been spent on pleasing donors.