We have to remember that Bethlehem had perhaps 300-400 residents at that time. As Herod had all boys under two years of age killed, the ‘masacre of the innocents’ may have only involved 20 little boys. In Roman (and much of ancient) history, that was a number of dead that was not even worthy of note, especially as they were children, not people of note.
How do you positively know this to tell us to "remember" the occasion?
Politically correct translations of the Bible (NRSV) have rewritten the story so all the children are killed (boys and girls). Of course, this would have made no sense in context of the culture at the time.
I’ve always noted the small populations of the ancient world and intend to make a study of it when I retire (someday). Most of our “towns” today would qualify as cities in those times - we’re just missing the walls and other protective structures.
For example the population of Bethlehem you mention is only half the size of the small town of 800 I grew up in in the midwest... the type you miss if you blink when your driving through.