because road bikes have about a 100 year head start on mountain bikes.

theres not a whole lot left to do on a road bike without going backwards. the are already the most effeciant machines on earth...

that and the uci requires traditional "double triangle" frames for racing. the bike companies make all their high end race bikes with double triangles and thats what trickles down.

in the 90's there were some pretty crazy tt bikes made (for lower drag) but the uci stepped in and insited on double triangles so now at first glance it looks like nothing has changed in forever.

in materials road bikes have been very progressive.