Because of the tight loading gauge in Britain, trying to cram two decks in would make it very small. It's been tried (once[0]) and they weren't able to make it fully double-decker, quick to load/unload, or especially comfortable.
I agree that they're fine in countries with larger bridges and tunnels -- Amtrak's Superliners are palatial in size -- but not for us. (Except probably for the Channel Tunnel rail link, which is built to French gauge).