Because when the Republicans are in power they fall in line, when Democrats are in power they fall in love.

To explain that last bit, Democrat candidates tend to have more direct goals, more local or state policies or projects they would like to enact. So everyone of them arrives in DC with something they are trying to do, and nothing usually happens as it requires the party to agree on it. Gathering the will to collectivly push past the GOP is usually too high of a barrier for most modest bills, especially when they are being extra obstructionist.