Frankly, I always just figured that in a country with only 62 million people instead of 312 million, it's harder to raise the kind of money it takes to support a 20-something-episode season.
That's pretty much what it is, the US studios throw much more money in to the production. A low budget show there is a high budget show here. Production gets behind then throw a second or third unit in and film until 3am to get it done, can't do that in the UK. The main networks budgets are in the Billions in the US, here in the UK BBC 1 has £1bn to play with, BBC 2 £400m, ITV1 £800m and Channel 4 just upped its budget to £600m and Channel 5 is around £200m I think. I think even your smallest network has a higher budget than BBC 1. When you get to cable networks you regularly hear of $1.5m per episode, where as you're looking at £150,000 per episode of the most expensive digital shows here, maybe a bit more of it's Sky.