Is this the relationship between rpm and thrust of a Turbofan?
The basic way jet engine and piston engine produce thrust on an airplane is the same. They accelerate a given amount of air which in reaction provide a forward force called thrust. The faster the fan or the propeller spins, the more air it can accelerate thus increasing thrust. There is a good answer there. The reason why they don't operate at the same percentage of available RPM is efficiency. It might be true that the thrust increase with RPM for any aircraft engine, but propeller have huge losses at high speed the propulsive efficiency going down dramatically. And this is mostly due to the tip vortex appearing at the tip of each blade as you can see in the picture below. As turbofan are enclosed in a casing, those tip vertices don't develop and you have less losses.

Now to your question, as you can see the curve start a 50% of the maximal RPM with a thrust rating very low. This is typically the case for turbofan where the fan design has been optimised for higher RPM. At low RPM the AOA ouf the blade is not optimal and the efficiency is quite bad.