Liberal policies of making everyone "equal" basically screws the people that work hard to support the system and rewards the ones that do little or nothing. Hard-working people eventually get sick of being "equal" and vote for the Conservatives. This ends the Liberal reign.
If we elected the liberals continuously we would have socialism. Socialism has been a disaster. If you remove all the conservatives like in USSR for example, you do not create an ideal society. The cons are essential for maintaining the balance.