British TV series are shorter because, from what I understand, due to having less funding and also more focus on quality over quantity. Less 'filler' episodes and more time spent on each episode.

I agree that it's annoying, but on the flipside, I wonder if they would be as good to watch if they made 20 episodes per season? The quality would naturally go downhill at least a little, and they would have less time to write more story.