A number of reasons, not least because if you put your napkin on a soiled plate, it is going to get far dirtier than simply placing it to the side. Why would you want to make more work for the person laundering the napkin than necessary - it just shows plain inconsiderateness of others.

Not to mention the person having to fish out your napkin from the plates, the fact it makes it harder for them to take plates to the kitchen as they cannot place another plate on top, the fact many do it after the main course, leaving them napkinless for any remaining courses, and having to see other people's napkins dumped on their plates is unsightly.

It really is very déclassé!