Because that has worked so effectively for the past 20 years.
In 1992, conservatives refused to vote for the RINO Bush, giving us President Clinton.
In 1996, conservatives refused to vote for the RINO Dole, once again giving us President Clinton.
In 2000, conservatives refused to vote for the RINO Bush II, damned near giving us President Algore (He won a majority of the popular vote. Thank God for the electoral college).
In 2008, conservatives refused to vote for the RINO McCain, giving us President Obama.
In 2012, conservatives refused to vote for the RINO Dole, once again giving us President Obama.
Do you see any sort of pattern here? Has refusing to vote for a RINO and letting the Democrat win ever worked?
Trying to convince the establishment Republicans not to nominate RINOs by electing Democrats is like trying to convince the environmental left that Global Warming is a scam by pointing out that every single global warming model that has predicted Global Warming has been wrong for the past 20 years.
The only way to get the establishment Republicans to stop nominating RINOs is to get a conservative candidate nominated and then get him elected by a landslide. Winning elections sends a message. Losing elections does not.
Typo, though RINOs are pretty interchangeable.