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.
The fight between Vader and Obi-Wan was just a backdrop for the dialogue. As stilted as it may be, it was some of the more significant dialogue in the entire trilogy. It's the verbal exchange between them and the sort of stare-down they do that matters more than the acrobatics. It wasn't about the dizzying action. There was a fair amount of kinetic action in the brief fight between Obi-Wan and Darth Maul, in contrast, but because the two of them had no real connection, it fell flat.
With Vader and Obi-Wan it was the final culmination of the bad student whipping the aging master, and Obi-Wan warning Vader that merely using his force against him would backfire in a big way. We needed to see that theme play itself out later to where Luke exhibits a more measured, compassionate use of power in Jedi that causes Vader to redeem himself.
Merely amping up the Obi-Wan vs. Vader fight to the point of two jumping beans bouncing off the walls is the sort of thing JJ probably would do if he had the chance to meddle, and it would be a perversion of the redeeming value of the OT, but that's how a lot of the audience perceive Star Wars these days, as nothing but thrill and no substance.