I agree, would be cool to see manufacturer’s go head to head. Issue is karting is a niche market- there would be no return on investment for them. The reason they do this in Motocross is to sell more cycles, they have a major following, TV coverage, and the market share is there for them to capture; win on Saturday night sell cycles on Sunday. That being said, karting is not a spectator sport nor does it have the appeal of the motorcycle.
As for the Yamaha, I remember when it was introduced (I was very young). They actually had a kart and all (they may have been thinking along the lines of motocross for the future of karting), in short time reduced to just an engine supplier. They have been around since the 70’s with very little change, tells me the money is not there for them as they really put no effort into major upgrades or a new ‘product’ line.
I prefer karting to motocross, it just is not as popular with a majority of the people (check Craig’s list and see how many cycles for sale versus karts). Wish there was a way to make it more mainstream, but for now the best we can do is support the people and companies that are making it work for us!