Why did so many different actors end up playing Quatermass?

Quatermass was a british rocket scientist in a pioneering series of early television science fiction on the BBC. There were originally three series (incidentally broadcast live to camera) with a forth added decades later by Thames Television.

The series was successful enough that all the original series were remade as movies. But only one actor played Quatermass more than once. Why were there so many changes?

The Broadcast and release dates with the actor playing Quatermass (sequence might not be quite right and the movies names were different in the USA)

The Quatermass Experiment (TV 1953) Reginald Tate

Quatermass 2 (TV 1955) John Robinson

The Quatermass Xperiment (movie 1955) Brian Donlevy

Quatermass II (movie 1957) Brian Donlevy

Quatermass and the Pit (TV 1958) Andre Morell

Quatermass and the Pit (movie 1967) Andrew Kier

For completeness (but less relevant due to the long delay before these were made):

The Quatermass Conclusion (TV series and shorter movie 1979) John Mills

The Quatermass Experiment (TV remake broadcast live 2005) Jason Flemyng

Best Answer

There are several reasons:

Reginald Tate died in 1955, roughly two weeks after it was announced that the second Quatermass series would be created.
John Robinson was not available for the third Quatermass film and he also had a great deal of difficulty during the second film with the dialogue. Brian Donlevy was a major American star at the time and the next Quatermass project was aimed at an American audience.
Brian Donlevy was a major American star and he was bankable for the Quatermass theatrical release. The film was a finacial success and Donlevy recreated the character in a sequel called Enemy From Space. He remains the only actor to have portrayed Quatermass twice on film.
Andre Morell was the first choice to have played Quatermass in the original series, but declined the role. He then played the role in the third BBC series and he is best remembered as being the "definitive" Dr. Quatermass. However he turned down the 1967 film role when it was offered to him.
Andrew Keir was selected for the 1967 release Quatermass and the Pit (5 Million Years To Earth in its US release). He was apparently not offered the role for the 1979 version and he died in 1997.
John Mills was offered the role but was reluctant to take it until he was convinced by his wife. Although it remains his most famous television role, Mills went back to working as a film actor after it was completed
Jason Flemyng - Was offered the role due to his exposure from other works. All of the other actors who would had previously played Quatermass were deceased or retired (except for John Mills, who died later that same year) when the production was created.