His parents were not US citizens... He is what is called an anchor baby. He is a citizen, but not a natural born citizen.
Cruz is the only one not on technical US soil. I wish he was ineligible but think that the courts would side with him by virtue of his mother's citizenship. Just like children of ex-pats are US citizens even if they are living in Paris at the time of their births. It also means that Churchill could have run for President as he was born in England but his mother was still a US citizen.