Because it's a 12 round fight. Lomachenko would have been prepared and maybe even expected to lose the odd early round or even take a round off later in the fight if need be. He had a challenge in front of him and passed it with flying colours.
He kept flicking his right hand out even when not aiming to land a punch, having a long arm in your face with the threat of a straight left down the pipe is not easy to overcome when giving away that reach.
He has to have studied footage of Lomachenko's only defeat in the amateurs. Selimov (as do a lot of Russian boxers) does exactly that as a rangy, counter punching southpaw.
Sometimes boxers and trainers overcomplicate things, trying to use some elaborate method to beat a fighter. If you have physical advantages use them. Lomachenko might have the best footwork in the world but no feet are as fast as someone's hands. A fast jab and straight shots are all it takes to land on someone much shorter.