I can think of many reasons. They both have the impediment of "no court will take it," and esentially, it is up to each candidate to be honest. Cruz certifies he is NBC. The voters decide the question on an individual basis. Everybody is smart enough to figure out what NBC is, it isn't rocket science. See constitution, etc.
The rest of the reasons are political. Mud slinging is a tricky and risky craft, sometimes the mud comes back, sometimes it makes your opponent stronger.
Mud or tar....
"Are you deaf or just rude?" demanded Brer Rabbit, losing his temper. "I can't stand folks that are stuck up! You take off that hat and say 'Howdy-do' or I'm going to give you such a lickin'!"
The Tar Baby just sat in the middle of the road looking as cute as a button and saying nothing at all. Brer Fox rolled over and over under the bushes, fit to bust because he didn't dare laugh out loud.
"I'll learn ya!" Brer Rabbit yelled. He took a swing at the cute little Tar Baby and his paw got stuck in the tar.
"Lemme go or I'll hit you again," shouted Brer Rabbit.
The Tar Baby, she said nothing.