Marple is broadcast by ITV, I believe...
We don't have long 'seasons' like they do in the US here in the UK, I guess through the lack of money being made available for the longer series in case of falling viewing-figures. Also, if the programmes themselves are longer (1-2 hours), the time taken to film is also very much extended over the 42mins given to programming in the US (generally). The actors tend also to be doing other work and are not always available for elongated runs of the same programme.