ella, here are some thoughts (without any specific order):

Political/Protest songs have always been just a part of folk music. There always have been those as well as love songs, working songs, drinking songs, ballads (with or without historical content - which is sometimes disguising political content of the latter day. Also one can question if the past is really irrelevant -it might contain the roots for modern conflicts), funny songs, childrens songs, dance tunes etc.

I don`t know too much about the people you mentioned but the aforementioned Sands Family - esp. Tommy Sands - and Brian McNeil still do write political songs. As do others. Or musicians who are not folk-musicians. Protest is not limited to folk and pop/rock reaches bigger audiences nowadays.

Maybe audiences became tired of it. Political songs have a much shorter decay time than love songs etc.. People are much more sceptic than they have been before when it comes to political statements. Also Society has changed quite a bit, bonds to organisations like political parties, trade unions etc. became weaker.

Best wishes
Ernest