Given this paragraph about the movie 'The Bridge on the River Kwai', why did the writers have to work in secret?

It was initially scripted by screenwriter Carl Foreman, who was later replaced by Michael Wilson. Both writers had to work in secret, as they were on the Hollywood blacklist and had fled to the UK in order to continue working. As a result, Boulle, who did not speak English, was credited and received the Academy Award for Best Adapted Screenplay; many years later, Foreman and Wilson posthumously received the Academy Award.[4]

They had to work in secret because they were on the Hollywood Blacklist for presumed involvement in the communist movement.