Why do the riders keep the same numbers, regardless of who won the championship last year?

So they can sell lots of T shirts with 46 on them ….

fair point 🙂

Is that all it is then, better from a merchandise point of view?

The guy who wins the premier class has the option to run the number 1 plate, many don’t as they are often highly superstitious about numbers. I believe as well there are some numbers that aren’t issued. It’s not just Moto GP that does this with numbers WSBK has done this as well, Biaggi is even touchy about what font his number is in.

Lots of series allow drivers to stick with their numbers -NASCAR is the best example.

I used to choose my lottery tickets based on favourite Motorsport numbers.

and they retire numbers from some classes when famous racers leave.