Why has Cruz sealed his birth records?
At some point, he will produce something known as a Consular Report of Birth Abroad, or CRBA (also known as an FS-240). This document is, as a matter of law, evidence that Cruz was made a citizen at birth, solely by the operation of an Act of Congress. In short, it is evidence he is viewed as "naturalized" under the law, even though we don't make infants participate in a naturalization ceremony. For good reasons, I suppose.
Cruz will probably pawn this off on a gullible public as his US Birth Certificate.
At any rate, other than a CRBA, his record have been produced. He's open about being born in Canada - except most people didn't know until last week that he was born in Canada.
Birth records are not public records, so the fact that this material is "sealed" is it's natural state. The records can be used to perpetrate identity theft, so it's good they are usually "sealed."