They chose a DeDion because it gave a better road holding compare to the independent rear suspension. If you take a look at any period magazines from 70s with the comparison between BMW 5, Alfetta and FIAT 132, you'll find that Alfa had much more superior handling.Strange since my 156v6, now sold, used strut based independent rear suspension. I cannot think of a more compact suspension design. Yes they are not the same, as the Chapman strut system uses the drive shaft as a link, but it is still extremely compact, or could be.
But your other comment, seems to be the reason and does make sense for being the cause as Italian automotive engineers have always been slow with change. Drums to disc, engine placement, independent rear suspension, v12 to v10 in F1 and so on ... always the last
For Busso and his team the most important were a not changing camber of the rear wheels under any circumstances and permanent contact of the rear wheels with the road. Cars with independent rear suspension could achieve similar stability only by the end of 80s.
Sent from my motorola one vision using Tapatalk