Availability and cost was the main reason. Back in 1962, when I got my mint No5 for $18, a sporting rifle like the M70 Win was selling for around $140-150. The old Winchester lever guns,which most hunters had used prior to WW2, were put in the rack due to lack of ammo and the fact that a lot of the old black powder ctgs were not much use any more as the country opened up and shooting distances increased.
The WW2 vets formed a new hunting community in the 1950s with a big demand for inexpensive sporting rifles.Sporterizing LE's and others,like P14/M17s,Mausers and Springfields was all the rage back then and this was a sensible way to go when you considered that monthly wages were $300 or so.
One of the main drivers for the US Gun Control Act of 1968 was the desire of various New England congressmen,like Dodd and the Kennedys, to protect the sporting arms industry in their constituencies from the flood of low cost surplus guns which were saturating the sporting community in the late '50s/early '60s. The JFK asassination provided the momentum to get this legislation through.