Hydrolastic as used by BMC were costly to produce and they were also heavy. (I've done my fair share of lifting the displacer units in and out of my 1800). Also rubber has a finite life which means we're all on borrowed time with them. British Leyland moved onto Hydragas with the Allegro, Wedge 1800/2200s and much later with the MG-F. Development of cheaper but very effective rear suspensions such as the VW Golf meant cost and weight savings.