Because they made assumptions that were inaccurate. For example, in refusing to apply the bill of rights to the States they assumed that the States would preserve the rights of its people.
The founders were wrong. As Barron v. Baltimore demonstrated elegantly. And Jim Crow. And many, many other examples. The States violate rights constantly. The founders were quite simply wrong.
Again, that is opinion. The constitution was designed to limit the authority of the federal government over the states.
No, that's the explicit findings of the US Supreme Court. They found that while Baltimore had clearly violated the rights of Barron.....they could do nothing about it as the federal courts weren't empowered to apply the Bill of Rights to the State of Maryland.
The assumptions of the founders, that the States would preserve the rights of the people, was found to be factually inaccurate by the Supreme Court.
And it is these inaccurate assumptions that render the founders judgment flawed and in need of correction. Which, of course, we did.