Love 'em or hate 'em why don't you see motorcycles with them?? just a thought 
It might be something to do with the inability of people to transfer plates from bikes to cars so making a 'personal' plate on a bike next to worthless.
FYI A similar ban existed from commercial vehicles but when Princess Ann (or Margaret - I forget) wanted a particular plate off a milk float suddenly the regs disappeared.
It may be that money has overcome the bike ban too, but I don't think so.