Bill Nelson didn't vote with his party and Rand Paul was a coward. These are the reasons this bill failed today.
You could argue that he is a fool, making the perfect the enemy of the good, but there's nothing here to suggest he is being cowardly on this issue. It's not only cynical to suggest Paul voted the way he did to sell out for his political future, it's also a misreading of the political landscape. A more accurate cynicism is to note that Republicans generally had little problem with an intrusive security apparatus so long as there was a Republican president, but that with a Democrat in the WH a Republican seeking to get support from the base risks next to nothing opposing the NSA.