It looks like the Glacier designated backcountry campsite and itinerary system was established in 1973.
See page 13 here:
https://scholarworks.umt.edu/cgi/viewco ... ontext=etd
"The designated campsite system was adopted in Glacier National Park in 1973. The system and Glacier National Park itself were eventually singled out and cited as an example of excessive agency control over its backcountry users (Merriam and Knopp, 1976). This criticism plus user perception research that showed freedom of choice and spontaneity as being important to many people's wilderness experience, prompted Park managers to examine the possibility of a limited and experimental zone-type reservation system."
Then, in 1975, the park established the Nyack camping zone to offer some dispersed backcountry camping. They chose it because of the more difficult access and lack of good fishing lakes and other focal-point destinations that would lead to crowding and over-use.
This paper at the link above was a study by a Master of Forestry candidate at the University of Montana in 1981. He studied how people were using the new Nyack dispersed camping area in 1976 - 1981.
"It was found that users supported the zone-type camping system. However, they did not select dispersed campsites that were beyond sight of the trail. Instead, virtually all users camped in the traditional, or in several newly formed campsites. Nearly all of these campsites were readily visible from the trail. The Wilderness Zone did not disperse use nor did it reduce resource impacts. Extensive resource impacts were probably avoided because of extremely low use levels."
There's some really interesting stuff in this paper about all of the studies that led up to Glacier and other parks establishing more stringent backcountry camping regulations. Yes, Glacier's system is more stringent than other national parks....but in my view, it's a good thing that helps to protect the Glacier backcountry and give us a better, unique experience.