The only answer anyone ever seems to be able to offer to that question these days is, "Because the Democrats are worse." However, the problem with being the lesser of two evils is that you're still an evil.
Well, what about the GOP's principles?
What about them?
Name something that the Republican Party stands for today.
Low taxes? We've won that battle so thoroughly that 47% of Americans don't pay income taxes even though we have a trillion dollar deficit. Meanwhile, many of the richest Americans are comfortable enough with what they're paying to the government that they're supporting Democrats who are pledging to raise their taxes. In other words, we're a victim of our own success.
Law and order? Americans in bad neighborhoods who are suffering terribly because of gangs and drugs are apparently of no interest whatsoever to any Republicans in charge of anything judging by the amount of time they spend discussing this issue -- which is close to zero. Additionally, the biggest law and order issue on the table today is illegal immigration, where the Republican Party is working hand and glove with the Democrats to reward 11 million people who are here illegally with amnesty.
A strong defense? What good is being strong without being smart? Most of our military "allies" are nearly useless in a fight, we're giving hundreds of millions of dollars to nations that hate our guts, and the Bush Administration put us a in a terrible situation in Iraq by invading without having the slightest idea that our troops would end up policing the country for nearly a decade. Most Republicans in D.C. seem unfazed by those problems.
Small government? If we really believe in small government, why does it keep getting bigger even when we're in charge? It certainly did under George W. Bush, who brought us Medicare Part D and TARP while doing nothing of significance to reduce the size of government.
We're the competent, responsible adult party? Compared to the Democrats, yes. Compared to the Republican Party of the Reagan or Gingrich era, no, we're not. When's the last time we pushed term limits, SERIOUS anti-corruption legislation, or tried to force members of Congress to obey the same rules the rest of us have to live under? We just ran a "Mr. Fix-It" candidate whose get-out-the-vote software crashed nationwide on election day for God's sake. It doesn't get much more incompetent than that.