Wikipedia: After being elected to the Senate from Kentucky in 2010, Rand Paul already filed for re-election in 2016, back in 2011. By Kentucky state law, he cannot run for re-election to the US Senate in 2016, if he is the Presidential or Vice-Presidential nominee for either major party. He likes being in the Senate and the modest fame he has, and may have decided that it's more important to secure his seat there than please his father's movement of libertarians and peaceniks, who don't have much influence in Bible-belt Kentucky. Christian Zionists are strong in Kentucky, and Rand Paul's wife is a committed fundamentalist christian Zionist. She brought him into her church, dispensational Baptists I believe.
He cannot afford to alienate her and the christian Zionists, the Republican base in Kentucky. They're probably as strong there as in South Carolina.
He knows that Adelson and the neocons would never countenance his being even the Vice-Presidential nominee, so most everything he says and does can be interpreted as contributing to his re-election to the Senate from Kentucky for 2016.
Strange that he gets elected in the first place by association with his uncompromising father, and re-elected by repudiating his father's stand in favor of the Iran agreement.
A pol's gotta do what he's gotta do.