long time between play and film?

I have fond memories of seeing this play at the Edinburgh Fringe Festival in 1979 or 1980.
I remember it was written by the writer of THE LONG GOOD FRIDAY and was well reviewed at the time but I wonder why it took so long to be made into a film.

People will say the racist police angle is still topical,and sadly it is but the script will confuse younger viewers with its comments about the novelty of female newsreaders and although we still have racist police I hope the vast majority of police are not like that nowadays.
So it is a period piece,another one of these British films that nobody sees in the cinema and then it appears on BBC at 2300 on a Tuesday night,I wonder why the makers bother?