The reason is to be found in the shape of the early blades, of whom are designed to distribute the same lift on each diameter.
As you can see in the document the blades thicken up to the inside as seen from above
Not the case for the blades of many older helicopters. They were very simply made, with a constant profile along their length.
The Whirlwind, which I flew very early in my career, was reputedly able to enter VRS at a rate of descent not much over 300ft/min. The blades were made from a simple, hollow metal spar, with metal pockets bonded on to form the constant blade profile along its length.
My present aircraft's RFM tells me to avoid rates of descent of over 900ft/min at low speed.