Obi-Wan won because of Soresu not the environment. Soresu is meant to stretch the fight out until an opening becomes vulnerable which it did and Vader lost. It doesn't matter if he was being pushed back if you can't pierce Soresu's defense then you lose.
Vader was hammered by Luke who never had time to learn saber techniques to mastery. Galen Marek tears Vader to pieces in the first game in their duel. Vader resorts to trickery and underhanded tactics compared to openly facing Starkiller in the second. Had Marek been a real apprentice he would have killed Vader and claimed the title of Master in record speed for a rule of two sith.