The speed of a helo is almost always set by the stall propensity of the rotor. The less remaining thrust on the rotor in a hover, the less the amount of thrust available to overcome retreating blade stall on the "downwind" side of the rotor in forward flight, and thus the lower the Vne.
It is also a fact that rotors want to be near stall to be most power efficient, so that the early recip helos all had to be sure they squeezed out all the hover lift they could just to get airborne. In other words, the blades were purposely made with much less chord, so they had to be operated at a higher pitch angle in a hover, and therefore at a better L/D. This thin chord rotor stalls much sooner, at lower airspeed. Why just recips? Because the recip engine is so heavy for its power that a recip helo typically spent 10 to 13% of its max gross weight on the engine (vice 4% for a turbine), so it has to pinch pennies when it comes to power, or it won't carry any payload. A turbine helo has way too much blade chord for hover (it throws away power) because it needs that chord for higher speed forward flight. This meant that recips were always designed to be near stall in a hover, making Vne very low in airspeed. That is why the Vne of almost all recips is at 100 knots or so, and the earliest recips had Vne of 50 to 60 knots.
A second contributor is that cruise drag is overcome by the rotor by using more thrust, so that a very draggy helicopter needs even more rotor thrust to pull itself thru the air, thus making its Vne even lower. The Fletner is especially draggy, so it would eat up rotor thrust (and therefore speed) fighting the drag.