Why didn't either one use a force push, jump, or speed in their final battle? Were their powers no longer strong enough to use these in battle? It could have potentially been a match deciding move to use a force push or speed in this battle...
First, the battle wasn't that long. It was basically a chat and then Kenobi kept hitting Vader's lightsaber. It was a very calm fight - If we can call it a fight. When Obi-Wan sees Luke he basically suicides and becomes one with the force. So, I don't even think It was a real fight. Obi-Wan was aged and Vader probably thought he could easily defeat him.
Also, don't forget the movie is from the 80s - we don't have acrobatics like the prequel ones or scenes like those. Hope I helped you
Novelizations repeatedly showed that lightsaber combat is more about using the Force than swinging around a blade.
Episode II explicitly ran into the "well, let's stop throwing fireballs at each other" with Dooku vs. Yoda.
I happen to think when Jedi/Sith fight, they are constantly using the Force to assess their opponent and to sense any and all threats. As stated in other answers, however, since Obi-Wan had bigger plans and Vader assumed Obi-Wan would be no match for him, there was no need for acrobatics or things like Force throws. But I wouldn't assume that simply because you don't "see" the Force, it isn't there.