Let's start with what etiquette is and is not. So first, a definition:
Etiquette: –noun
1. conventional requirements as to social behavior; proprieties of conduct as established in any class or community or for any occasion.
2. a prescribed or accepted code of usage in matters of ceremony, as at a court or in official or other formal observances.
3. the code of ethical behavior regarding professional practice or action among the members of a profession in their dealings with each other: medical etiquette.
—Synonyms
1.
Etiquette, decorum, propriety imply observance of the formal requirements governing behavior in polite society. Etiquette refers to conventional forms and usages: the rules of etiquette. Decorum suggests dignity and a sense of what is becoming or appropriate for a person of good breeding: a fine sense of decorum. Propriety (usually plural) implies established conventions of morals and good taste: She never fails to observe the proprieties.
Ok.... So let's dig in a little - Etiquette is the formal rules governing behavior in polite society. It's basic rules like how to properly announce someone into a room (using titles, etc), how to introduce one person to another, and the like. It's also why no one dances until the bride and groom dance, and a LOT more.
While some rules of etiquette are tied to historical references - like keeping the bottom button on your coat unbuttoned so that it doesn't tear out while riding a horse, or why a bride is located on the groom's left arm, or why bridesmaids all typically wear the same dress or why a bride wears white (it has nothing to do with 'virginity', despite a lot of people's ideas to the contrary) - others are not.
Some are just the proper way of doing things. The 'rules' of proper communication, etc. It's what people EXPECT in polite society, and not understanding the rules is what makes a lot of DJ's appear cheesy, makes people uncomfortable when they speak, or worse. Well, that, and their trying their best to imitate Ted Baxter...
Anyway... to answer your question. Congratulations is for the groom.
Here's why:
–noun
1. the act of congratulating.
2. congratulations, an expression of joy in the success or good fortune of another.
–interjection
3. congratulations, (used to express joy in the success or good fortune of another): Congratulations! You have just won the lottery!
A groom, is congratulated because he has 'won' a bride. Even in the wild, brides (female 'mates') are 'won over' by something the male has 'done' or 'possesses'.
A bride on the other hand didn't 'win' the groom because traditionally HE sought HER (hence the term 'courting'). So the proper etiquette in communicating your desires for a bride, typically in the receiving line, is to tell her 'best wishes'.
So, to answer your question - it's the proper communication based on the rules of etiquette in a polite society.
As for knowing this stuff, I know a LOT about various cultures, their courting rituals, wedding traditions, etc. and I also try to study the rules of etiquette on a fairly regular basis. The result is that I have people coming up and asking me for my card, not because I play music, but because they feel 'comfortable' with my methods and how I present information to them. It's what they EXPECT - even though they might not outwardly realize it.