I imagine it's because their use is evident (no need to advertise what they do, since they've been common in households for the past half century), and they've likely peaked in terms of function. You can't really improve much beyond what we have now, the last great advancement in the microwave was probably the turntable for consumer units, and the moving fields that commercial units have. You're limited in power output by the outlet you plug it into, so "faster cooking" is a no-go (unless you stuck a 240v plug on the end, and good luck getting that to fly with your average consumer in the US, we already find those annoyingly sparse for dryers and ovens and such) -- what else can you innovate to differentiate your microwave from every other microwave on the market?