Part of it had to do with Alec Guinness himself not being the most physically adept aging actor at the time that was shot. Lucas also said that he wanted to demonstrate that Obi-Wan was in his waning years and not up to speed and Vader was a part-machine shadow of his former self, both men slowed and hobbled by their physical conditions at the time of the Death Star duel. I think it was always his intention that the lightsaber battle in the first movie would be kind of clunky and confined to that one section of the landing bay, in part to give it a more claustrophobic feel with Obi-Wan cornered and not really able to get away even if he wanted to.
Just my two cents' worth based on things I've read and heard over the years. I think the battle is rather effective considering Obi-Wan was just going to sacrifice himself and become one with the Force to provide Luke and the others time to escape. And Vader seems more of a tragic character when you see that he's no longer the athletic, jumping and lithe Jedi Knight he once was.