The very first lightsaber duel that we saw in film in 1977 was between Darth Vader and Obi-Wan Kenobi. At the time it was something we had simply never seen before, and it stunned audiences. As the Star Wars saga has progressed, so too have the lightsaber duels. Each has had a dynamic of it's own. In TESB we saw a Jedi trainee taking on Darth Vader. Vader toyed with Luke, and the duel built emotionally as Luke came to grasp how out of his depth he truly was. In ROTJ we saw more of a duel of equals. Luke is more mature and has different intentions, and is trying to appeal to the good that's still within Vader. Neither is an exceptionally gifted duelist, but the emotional stakes seem to have been raised.
In TPM we finally saw the Jedi in their prime against a Sith with amazing physical capabilities. That continued on into AOTC with a duel between masters of The Force. And that continued through ROTS as we saw the duel that was the climax of the fall of the Jedi, and the rise of Darth Vader.
So why was the lightsaber duel of ANH the way it was? There are plenty of practical reasons why. And now
this Uproxx article discusses some possible story driven reasons (based on material released AFTER Episode IV, mind you) why the A NEW HOPE duel may have transpired as it did.