It was probably a simple oversight. After studying the Constitution so extensively, they seem up space for elasticity and change everywhere. Perhaps they didn't expect guns to change as they did. Mass production and global availability also weren't factors in their world.
I think I'm going to spend tomorrow taking an in-depth look at the Magna Carta and trying to find/not find parallels. EDIT: turns out the Brits have been able to largely slice and dice the MC to fit the times. But it took them several hundred years.
The Brits have a really cool system based mostly on precedent and intentionally don't have a central constitution. I took a class comparing their, Japan's, and our own constitutional systems. At first when hearing of Britain's system from an American perspective, it's incomprehensible. We've been entirely conditioned to believe that everything their system is based on will fail. We can't understand not having central documents without there being rampant corruption or solid written rules to dictate things. Every aspect of their politics is what we're told is wrong. I suppose it makes sense as it's what our ancestors intentionally left, but still being so opposed to it 200 years later is astounding.