"Why won't the manufacturers make this particular locomotive?" is a constant refrain.
Trouble is, even if they release a different number model every 6 months, many classes would take years to work through. You might be luck, but you're more likely to have given up model railways due to old age before your pet loco appears.
Renumbering isn't hard though. There are plenty of transfer makers out there producing superb products to help. All that is required is a little nerve to remove the old number and replace it. The help, I've written up the process for the latest issue of the Hornby Collectors Club magazine.