Because "back in the day" women were considered far less capable of anything the men were working on, and vaulting across the width of the horse required less distance to clear the horse. The skills were also well suited to that set up, as the big deal skill back in the early 60's, if I'm not mistaken, was a handspring vault...... with no new vaults in clear view. The springboards were also much different than today's. They resembled current boards with a curved top surface and to base legs, but the spring action was provided by a wood leaf that resembled an elongated "S" laying on it's side with the curve removed from the top and bottom of the "S" shape.
The irony of it is that some wood boards were better than the others, and everybody would want to use "the good board" brought to the meet by the lucky team that actually owned it.