I think that Romanzo criminale has a great success because the story behind is good... as only a true story can be
The story of italy in the 70's is very interesting also today; italy was one of the most important 'centers of the world' for many many things
the eyes of the world saw what happened in italy, and Banda della Magliana had a role in almost all the most important situations happened in italy about 1975/85, so the story of Banda della Magliana is also the most interesting story about italy in the last century out of WWII
70's in italy were very dark years... full of mistery, violence, danger, a lot of persons killed for nothing
but story of italy today is really stupid, nothing interesting to tell, so also directors watch in the past...