The Show Is Produced Outside of the U.S.
International shows have a different concept of longevity than stateside programs. Most of them aren't built to be older than your dad (except Doctor Who). Perhaps it's the demanding audiences of Britain and her former colonies; maybe it's the artistic temperament of showrunners hailing from overseas.
Whatever the cause, we've seen many of the best shows from elsewhere including Slings and Arrows (Canada), The Office (U.K.), Summer Heights High (Australia) and Absolutely Fabulous (U.K.) bow out after only a handful of truncated seasons. We know that you TV Angophiles are plentiful and growing in number with each new episode of Downton Abbey. We encourage you to treat your fandom of these shows just like your semester abroad: Don't expect too much and you won't get your heart broken.