I guess it's what folk get used to! In the UK the main terrestrial channels have so many programmes to schedule at various times throughout the year that it wouldn't be practical to make series' too long! for instance the BBC drama budget is only a percentage of the licence fee. Out of the licence fee also comes funding for Sport, Entertainment, arts, movies, Reality, religion, factual, current affairs, news, and so on! it also has an obligation by law to show a variety of different programme genres as well as fulfilling it's public service remit. Therefore limiting its drama shows to say 6-12 and entertainment shows the same amount it is able to find room to cater for all, and on occasion it is also obliged to show religious ceremonies that wont even rate!