Because they have poor aero, poor components and poor comfort.

But I have seen a rider on an 89 mountain bike smoke club riders on carbon frames. The MTB had fast slicks and drop bars fitted, so it was aero and the RR was low. It was heavier, but on the flat that didn't really make a difference. Regretably, it didn't have "Dark Angel" wheels like this one:



Why did it hand out said smoking? The rider was faster, and the road the race was on rewarded fast braking, high speed turning, and the ability to roll over a second rate road surface with low RR. Low weight is just one factor in bicycle performance - which is why pros often ride cross bikes on harsher Roubaix stages despite the weight gain over a pure road bike. Weight is what marketeers get people to look at, because its the most profitable factor - for the marketeers - to reduce. Getting RR lower will normally benefit you more and cost a lot less, but it doesn't get pushed by the industry because it is so cheap to do.