It's not just the Constitution, it's the entirety of the American system, which seems designed to use "checks and balances" as veritable roadblocks to slow everything down.
I agree that - in theory - it's great and very smart, because if the government was indeed designed around truths that are self-evident (i.e. inherently "true" and immutable) than everything should settle at the mean of those truths.
The problem is that when the country was being formed, incredible compromises to those self-evident truths were made left, right and center. So we ended up with a country that claimed that all men are created equal but did not actually adhere to this.
Further, the Constitution & Bill of Rights is full of things not related to these core truths. They tend, often, to be functions of their time. Those are the things that should be open for change, and the core truths should be canonized and unbreakable.