Because they both come from the same linguistic root and mean "moral or cultural decline as characterized by excessive indulgence in pleasure or luxury".
Apparently the Latin root of decadence means "to fall down" and is a synonym for excess.
The implication with food is something that's a bit too much, or a bit more than necessary, isn't it? So that's likened (in exaggeration) to times and places when people did everything on a grand, unsustainable scale, like say, the aristocracy in pre-revolutionary France. I think you're supposed to get a bit of guilty pleasure out of having something you don't need, a luxury.
Food hundreds of years ago was for sustaining people to live.
Today one could hardly say the same for what we have invented as ''desserts''.
That is a food we should all stay away from....but in 90% of cases can't!
It is of no nutritional value, and will put on weight.
That is 'decadence'.
Having something of no real need or value to your life.
Something that is pleasurable, at a cost(e.g. calories in this case and great tasting).
I disagree Jonaj. When you see the menus concocted for medieval banquets, there was plenty of surplus food there and several pudding courses! And for real decadence, just look at the Romans.... they really knew how to party! Overindulgence and moral decline are nothing new.