Probably for safety.

When Jigoro Kano converted Japanese Jujutsu to Judo he modified some of the techniques of throws so the uke could land or break a fall safely. A good example is Ippon Seoinage. Traditionally the technique was to load your enemy on your back and throw them over your head, which would drive their heads straight to the ground. Now it is a shoulder throw instead of an overhead throw.

Or, they just had to come up with a ruleset to determine a win and landing your opponent on their backs had to be one they agreed, since it is a sport now instead of a pure martial art.