The Portacatena was marketed more to serious racers; they could shift onto the Portacatena while still riding and be immediately ready for a wheel change when the support vehicle pulled up. But it came on the market around the same time six and seven cog clusters were replacing five cog clusters. The portacatena occupied space that an additional cog would need, and ultimately The Market decided that more cogs were more useful than the portacatena.

But that doesn't explain why chain hangers on the seat stay pretty much disappeared after the mid 80s.