Because the American shows are investing so much more into getting a show off the ground, they need so much more return to make that money off.

Think of it this way. Say it costs 5million to build the sets and do all of the stuff required to get the production up and going. Then each episode itself costs 500k for all the stuff needed for that specific episode. If you make 6 episodes, you spent 8mil(5mil set up, 6x500k=3mil, totaling 8mil). This means you need to sell 1.3mil in advertising per episode just to break even.

But! If you make 22 episodes you’ve spent 16mil(5mil set up, 22x500k=11mil, totaling 16 mil.) Now you only need to earn 727k per episode to break even.

The traditional american broadcast tv economy lets them charge the same to advertise on a show whether there is 6 eps or 22. So the larger the initial investment, the more benefit there is to spreading that initial investment across more episodes.

Because the BBC was publicly funded(and doesn’t invest nearly as much money into productions), they don’t have the same incentive to maximize earnings by spreading their expenses across as many episodes as they can.

Cut to decades later and the cultures behind tv production have grown in wildly different directions on either side of the pond.