The reason given for the change is that the old style is more combative - we do not trust our opponent, so we keep the sword hand ready until the last possible moment, and then free it at the earliest possible moment. For modern kendo now that we are all just practicing together, we have a more trusting bow with both hands together. Similarly if you bow keeping eye contact until the last moment, then restoring eye contact at the earliest, this is a more combative method.