Why creep dont affect terran and protoss ground enemies?

It could be a great advantage if the Zerg creep decrease the move speed against Terran and Protoss ground units.

It would be, but I think it was bad idea to have creep give speed bonus in the first place. It should been default to all Zerg units.

How many % is the speed bonus?

I say between 10 to 15% that give Zerg 40 to 45% speed difference units that has cliffwalk should be inmune to it