Because they know that the real battle will be in the states. And the Republicans also know that they don’t need the White House.
While the Democrats must win the presidency to act as a stop-gap against an overwhelmingly conservative Supreme Court that could last for generations, the Republican Party doesn’t have as much to gain by controlling the White House. Control of the Supreme Court and every other rein of federal power is firmly in Republican hands right now.
The 2010 gerrymandering of congressional districts makes it almost impossible for Democrats to win the seats that would truly represent the votes they garner. The Senate is not likely to change hands during 2016—at least not enough to give Democrats the filibuster-proof majority they would need to accomplish anything. The Roberts-led Supreme Court is perhaps the most flagrant, politically partisan court we have ever seen.
So the Republican Party is free to let the circus clowns provide the entertainment its base wants, ensuring that they will send in their contributions and turn out for the primaries as well as the general election. It doesn't matter much whether Ben Carson or Donald Trump runs as their candidate. If a candidate with independent appeal takes the nomination, well, that would be fine too. Not only is a win not absolutely essential, but if they did take the White House, then who would veto all their grandstanding about the Affordable Care Act?
They have already proven that they can prevent a Democratic president from governing. And that is all they want to do on the federal level, keep the government from actually governing. Oh, and transfer as much wealth as possible to the top one percent and to the corporate oligarchy. But the real action is going on at the state level, where not only does the GOP need to maintain majorities to control the 2010 redistricting, but all of the coffers have not yet been emptied.