Weight is determined by how fast it goes, and what it's made out of. Exotic materials drops weight, but drastically raises the price.
40 to 50 mph demands a chassis that weights at least 100lb if it's going to be able to take that kind of pounding dependably for years. Going 50 mph on a 100 lb bike requires very careful, intelligent frame design and engineering.
I had a Honda S90 once upon a time. It went 48 mph, and weighted 200 lbs. It wasn't very stabile in turns, and the drum brakes were like suggestions to stop. My Stealth weighs 116 lbs and goes 50+ mph, and rides like it's on rails in turns. The disk brakes are so powerful you can lock both tires at will. Both bikes have steel monocoque frames, but the Honda was a cheap stamping, while the Stealth is a very time consuming to make design, welded up from plate.
Until some new metal is invented 50 mph bikes are going to weigh at least 100 lbs or more depending on how clever the design.
