A few reasons for gas only  

First methenol burns almost in a clear flame making it hard to detect a fire until it's to late. Second is the fact that it burns very cool vs gas which lets you remove the HP sucking fan (a good thing) and because it runs so cool it allowed us to build big bore as well as high compression motors. The bigger the bore the thinner the wall thickness between piston and water jacket making heat. When the 467ci rule came out it removed the chance for big bore. Some tracks even went to a compression rule which ended the 12/13:1 heat making compression motors. So for those reasons the benefit of Methel gets smaller and smaller.