It wasn’t added for no reason, they didn’t want immigrants to be able to become President. They only required 14 years of residency in the U.S. to become President, so obviously they weren’t paranoid that someone who lived outside the U.S. for even most of their life would be some sort of evil foreign agent. It’s up to other countries to decide who becomes a citizen or not. Canada could make everyone in the U.S. a citizen now if they wanted. It’s irrelevant to someone’s qualifications. Did they leave out a part of the Constitution that might have said you can’t be a dual citizen for a reason?
They didn’t say anything about it because the concept of being a natural-born citizen of two countries would never occur to a reasonable person. It only occurs to people who want something so badly that they are willing to cut corners around the law to get what they want. I, for one, have had enough of that with the Clinton, Bush and Obama administrations, and I don’t want to do it again.