The ICAO led initiative for MLS was based on the predicted increase in air transport, the demand for instrument approaches, particularly CAT 3, problems of frequency overlap (runway density), and potential for commercial VHF broadcast interference.
Competing UK and US systems were demonstrated cica 1978/79 with CAT 3 / RNAV performance and integrity which are only just being matched by today's GPS.
The problems were competing political interests in the avionics market, cost, which the operators did not wish to have , and that the perceived threat to ILS was not imminent. The environment / noise lobby was weak, no money, and inability to pressurise the industry.
The GPS card was played as a stalling strategy, but was subsequently mis-interpreted as offering 'the world and more', which influenced the less knowledgable nations during the ICAO evaluation.
The threat faded into history although many of the concepts continued in the military, Madge etc, and specialist private installations e.g. Aspen.
Due to the limited computing power at the time (no FMS), curved approaches used a few 3D waypoints which were linked by existing LOC and GS capture modes in the autopilot. The end result was a very impressive smooth approach which enabled multiple azimuth changes or lateral offsets, and a choice of glide-slope angles and transitions, e.g. 4deg changing to 3deg at 1000ft' and vice versa, as well as straight in steep approach 4 deg.
A 4 deg autoland was demonstrated on a non ILS runway after a 30 deg offset 3 deg GS, transitioning to straight in and 4 deg GS final.