Civility

Many of the manners so cherished by some fall directly from there being a tradition of dueling in Western Civilization.

Elaborate manners developed to keep one from ending up dead from giving offense or taking offense.

Western Civ also used to have a tradition of shunning those who couldn't behave within the bounds of polite society.  The crass and the rude would find themselves unable to support themselves.

Several non final outlets for pent up behaviors were available, getting plastered at a bar being one of them.  "Oh, he was drunk," was an actual positive defense against boorish behavior that'd get you otherwise ostracized.

In essence, there were consequences to being rude, often severe, that kept people polite.  Those consequences also kept the rules about what was rude or not consistent.

Now...

There are few if any consequences to being rude.  What consequences remain are haphazardly and unevenly applied.  What constitutes rudeness is inconsistent, subjective and random.

If you really want people to be polite, you have to make following the rules more valuable than breaking them.  If you want people to follow the rules, they have to be aware of them.