IIRC Cruz himself said that he is now signing up for ACA, which means before when he was under his wife's plan he wasn't in ACA. So to stay out of ACA all he had to do was stay in her policy via COBRA. The political benefit would have been huge for him.
He's a smart guy, and I'm sure he has smart people around him. He had other options besides ACA, I have no idea why he went into ACA.