Actually, no "good reason" at all -- only uncorroborated rumors. You're making things up.
Cruz, of course, knew that he was born in Canada. But Harvard ed or not, he had no particular reason to be familiar with Canadian citizenship law, and would not necessarily have been aware of his standing -- which, in any case, was legally irrelevant.
No I am not making things up. His morther married a English man and moved to England she most likely applied for and was granted UK Citizenship. If so upon her arrival in Canada she as a citizen of the UK could have gotten Canadian citizenship in just one year. She was on the voting rolls, which requires citizenship, so she likely was in fact a Canadian citizen just like her husband.