But it has. He was a student of Obi Wan Kenobi, who thought he could train him as well as Yoda. He was wrong. Vader was seduced by the dark side. He helped the empire hunt down and destroy the jedi. That's all anyone ever needed to hear about it. Vader wasn't "the most powerful jedi ever" -- far from it. In the original movies he got beat by a barely trained kid and easily killed by the emperor.
The thing that disappointed me the most about the prequels was the wholesale adoption of the ridiculous lightsaber crap from ROTJ. The lightsaber was presented in the first two movies as being like a samurai sword -- a traditional, ceremonial weapon that might occasionally be used in duels or to punish peasants who offended you. It wasn't supposed to be a serious weapon of war and presenting it that way made the new movies even more cartoonish and ridiculous than they were already, and that's saying something.