Grammar usage

I'll venture a guess and say the reason is tradition that dates back to before Gutenberg invented the printing press with movable type.  Granted, there was movable type in the far East before that, but they weren't speaking English.

Before the printing press came about around 1450, every copy of something that needed to be reproduced had to be done by hand.  That includes invitations.  Abbreviations were commonly used to speed up the time it took to make each copy.  An example is one we still use today, Xmas instead of Christmas.  This abbreviation uses the one Greek letter X (chi) to replace five letters.  This abbreviation is known to have been used as far back as the year 1021.

So, back to the tradition part and my educated guess.  Marriage in those days had a great deal to do with the honor, pride, and status of families.  Wealth increased that status.  It was more costly to pay someone to spell out every word, and probably looked like the family was trying to be cheap if they allowed abbreviations in the invitation.  They want their daughter to marry the duke, but can't afford to spell out all the words?  What an insult! 

Marriage traditions have a very long history.

A wedding invitation is considered to be highly formal writing. In this style of formality, abbreviations are not used and years are spelled out.

Why? Because that how it has "always" been done.