The only reason I can see is they are a proven top seller and people ride then forever so to speak. They are at a price where almost anyone can purchase one. They are somewhat builtproof, I did say somewhat. Do the DOO and move on at least.
If they were to do an engine update to include 6 gears I would be it will include FI too. That jump would push the price of the bike into the BMW range I bet. Thus taking it out of the very profitable range it's in at the moment. For us and them.
It is a relatively cheap bike to buy and maintain, parts are readily available almost anywhere on the globe, so why mess with a strong working formula?
Yes even I am still looking for 6th gear it seems lately, would it be nice, sure. But I won't loose any sleep over not having 6th. If someone could redesign the gearbox for a saleable price, I may go for it. But I can just as easy change the sprockets to get close to the same thing. I ride solely on the street. Dirt riders already have the same choice, gear it down for better get up and go if they choose.
I think we always are asking for a better bike for the same money....it will never happen....IMHO.