Edelbrock wanted to expand and carry carburetors, and it was much more cost effective to purchase existing designs. Looks like both Carter AFB and Rochester Quadrajet were receptive to selling. What surprises me is that I see that Edelbrock has actually brought out new models in both series. And regardless of your feelings for specific brands, competition will give us a better product from all manufacturers.
I'll bet the Edelbrock is the more expensive carb? I know that the Edelbrock rebuild kit is a lot more money than the Carter, and both rebuild kits are the same as far as I can tell.