The reasons are very simple.
Read the article and pay attention using common sense.
Reality Check Constitutional Eligibility Divided Loyalties Need To Obey The Constitution - Birther Report Obama Release Your Records
Cruz is eligible.
It is up to the voters to decide whether or not his loyalties are in question.
I agree on Cruz's eligibility, as the US recognizes only citizens at birth and naturalized citizens. There is no third type. Since natural born citizens are clearly naturalized, that leaves citizens at birth as the only possible category under which 'natural born' could fall.
Though in fairness, for those not born in the US the issue is at least debatable. Take a look at the debates on the eligibility of Mitt Romney's father when he considered a presidential run. The general consensus was that he was eligibile. But the debates appeared legit. They're summarized here:
Romney s birth certificate evokes his father s controversy - Chicago Tribune
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Defining "natural born"
So what is a "natural born" citizen? The Constitution doesn’t specifically say.
In 2008, we reviewed research and polled several legal experts. The consensus was that someone is a "natural born" citizen if they have citizenship at birth and don’t have to go through a naturalization process to become a citizen.
If that’s the definition, then Cruz is a natural born citizen by being born to an American mother and having her citizenship at birth. (This same logic would apply to Obama, even if he were born in another country, which he wasn’t.)
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More at the link..
Is Ted Cruz born in Canada eligible to run for president PolitiFact