WHY did they change the end?

I saw this film on the TV one day, many years after having read the novel.

My feelings for the novel are somewhat ambiguous - maybe that's because I first read it as a child of twelve, and parts of it struck me as very "dark" (also I couldn't understand why a minor theft would warrant such harsh treatment even if Jose HAD been guilty of it - but it's definitely memorable. (How could it not be, with Cronin telling it?)

And it's the heartbreaking end (including the events that lead to it), while perhaps far-fetched and unlike Cronin's usual realistic endings, what "cements" the powerful impression made by the characters and their interaction - and Nicholas' emotional independence from his father.

Why, then, did the director choose a "happy end"?
I mean, I get it - it's contained in the very phrase I used: "happy end" - but was it really necessary?
Surely ten years after the end of the dark war days people would "handle" a more bleak end?

And I must say, I am somewhat surprised that Bogarde - never one to shy away from "darkness" - accepted the part considering the end.