When Cruz runs for reelection to the Senate, SOMEBODY is going to run an ad saying he defrauded the electorate last time by not disclosing he was still a Canadian Citizen. While it was well known he was born in Canada, NOBODY knew he was still a Canadian citizen.
Then Ted can spend the election cycle explaining how he, himself, did not know at the time.
That costs Cruz 5-10 points in both the primary and the general.