There's also just way too much crime for our English common law based system to handle properly, with a jury trial and all. Without going back to original details like executing those convicted of real felonies, which we're too soft to do as of now, not to mention our police/judicial complex is simply not that trustworthy, the current plea bargaining system is probably the best we can do.

And there's the minor detail that US crime has dropped dramatically from its recent peak; per the thesis of Arrest Proof Yourself, without a steady diet of "the clueless" being run through the system on petty crimes at worst, many police units and courthouses would have to lay off people, and that wouldn't do.