That and because the Republican party is simply there to keep conservatives in check. There is no opposition party.
If the Tea Party/Constitutionalists/Libertarians/Conservative parties could come together and organize a real opposition to the Republican sell outs, then they could easily defeat the democrats simply by shining the light of truth on them.
Of course if we keep relying on the worthless Republicans, then expect to see our country continue its drift to the Left.
True.
But there is only one way to do that: Take over the Republican Party machinery.
A third party would require recruiting from scratch, building and staffing an entire organization, plus creating a new donor network.
It would take years to construct such an organization and start winning elections. In the meantime, that would leave the field to the rabid moonbats of the Democrat party -- insuring the failure of the country an our efforts.