Hmmm, let's see.
Was there some kind of nation-defining event going on in the years just prior to the writing of the Constitution? One that might have involved lots of people and lots of guns?

Sure, but no more so than the King sheltering his troops in private citizens’ homes (or taking over the homes in toto.) That wasn’t important?

Certainly no more than the King (or his dominions) prohibiting public gathering or speech.

The King or his troops, at the time, could arrest anyone and imprison them without cause. That wasn’t important enough to say “Because….”

Likewise with the right to a speedy trial, the granting of bail, indiscriminate search. None of those met the threshold for having a prefatory clause in explanation?

(Here’s the answer: because the colonists eschewed having a standing army, so having a ready militia was seen as vital to the national defense. But as the need for a private militia faded, so does the need to have a privately armed citizenry - which is not the same as “confiscating all guns”, even though 2nd A advocates position it so in their false arguments.)