Because the colt grip frame is a spearate piece (held on by 5 scres..two on each side of the hammer, tow behind the tigger, and one ahead of the trigger), can clamp in a one pice (inleted) grip. Many were made this way, no screws t hold the grips to the frame bause it was one piece that was clamped into place when the grip frame was attached.
Is one "better" than the other? Not really....the two grip panel and a screw tye work just as well.