On one hand the show has time constraints each episode must take just between 20-25 minutes and one on one adds atmosphere making it more about the Pokemon. On the other hand they are a lot of filler episodes in the anime and team construction is part of Pokemon batting and by having say an entire tournament with one on one battles you're shorting a key part of the competition some say the most important part. Another thing is while the writers have time constraints the characters don't: their journeys are about training pokemon and with the type system a one on one battle is very unbalanced , a trainer's goal is to have multiple strong pokemon not just one.
If you look at the stadium in the 4 episode tournaments in BW that seems to be a pretty big stadium for such short battles and they only did 8 of them at most per a round. Seems a little unrealistic.
Paul and Ash's first battle(which was three one on one battles) clocked in at under five and half minutes including dialogue starting at when the first pokemon selection is made and ending when the ref declared the winner. So an evenly matched regular 3 on 3 battle could theoretically happen in under nine minutes even less if they don't pokedex anything.