One of the attractions of half frame was that it halved your film costs.
By the 1980s, print and slide film was relatively cheap.
In addition, when half-frame cameras were relatively prominent, "full-frame" SLRs were more of a specialty item and fairly expensive.
By the late 1970s/early 1980s, very capable "full-frame" 35mm SLRs were much more easily found and much more reasonably priced (e.g. Canon AE1).
And slide films were still very popular, but half-frame slides tended to disappoint.
In addition, 35mm rangefinders and point-and-shoot cameras were well advanced.