President Obama has nominated a highly respected, highly intelligent, extremely competent moderate Judge.  Judge Garland could have just as easily been nominated by a moderate Republican President.  McConnell, Ryan, etc are betting that Trump or Cruz will be the next President, they will hold their majority in the Senate and they will get a very conservative Justice for the Supreme Court.  They've so publicly committed themselves that it would be very difficult to change their position.  Perhaps they might consider that Ms Clinton (or on a long-shot Bernie Sanders) just  might be elected and they'll be faced with a nominee far to the left of Judge Garland. 

President Obama has offered a reasonable, compromise nomination.  Now we'll see how stubborn and foolish the GOP Congressional leadership decides to be.