i'll tell you exactly why:
Majority of Americans want a principled and trustworthy president. This is EXACTLY how obama won. "Hope and Change"...he felt trustworthy and that he would be different from bush, clinton, etc.
He failed. He isnt and wasnt different, he isnt trustworthy, he is a liar.
Enter GOP field. Name them all...cruz, bush, christie, graham, etc. etc.
Does the public actually trust them? Nope. Cruz is probably the most, but even your average voter is scared he's just a talker and probably more of the same.
Enter Rand: principled, trustworthy, etc. Shoot, people that dont even fully agree with him are on board that victory ship. Why?
Trustworthy.
What Rand says, you know and feel its what he's going to do. He isnt some random obama smooth talker. He'll walk the talk.
You wont get that out of ANYONE else in the field, and its the one thing that Americans want most: stop the lying.