I'm looking at the 100 people project and I am looking at clean airplanes. Why did we stop using a clean source of lift, the propeller?
Look into Propulsive efficiency (and maybe also this one). Turboprops have a speed of peak efficiency, which is around mach 0.6 or so. If you try to go faster, it gets significantly less efficient. A high bypass turbofan, on the other hand, is most efficient at a higher speed, maybe mach 0.8 - 0.85. So the simple answer is: we switched from props to jets because people wanted to fly faster but didn't want to burn a lot of extra fuel to do that.