A friend asked me this question the other day and I did a little research...
This fashion guideline is typically attributed to King Edward VII, the British monarch from 1901 until his death in
1910. Quite the gourmand, King Edward loved his food so much that the
royal tailors often had trouble keeping up with his ballooning figure. One
day, seeking reprieve from the confining constriction of his waistcoat, King
Edward casually unbuttoned the bottom button. At that time, the King set
the fashion trends and when members of his court saw his new look, they quickly
emulated it. The fad spread like wildfire and within weeks unbuttoned
bottom buttons were found everywhere.
We continue to honor the memory of Edward the
Wide to this day. Modern suit jackets and blazers are actually designed to
cutaway at the hips with the bottom button left undone. Buttoning it
results in unsightly pulling and bunching of fabric at the waist and
disapproving looks from your fellow gentlemen.
Probably the same reason they still put buttons on the sleeves that aren't functional.
Stupid fashion thing, really.
If the cut allows it, I button mine all the way. Otherwise, I deal with it...