[azernik]

The question isn't why they were phased out; it's why they existed in the first place.

They enabled aircraft to make fuel stops at undeveloped supply caches that happened to have a functioning seaport, but no long runway. This was necessary both because of the scarcity of paved runways at the time, and because the short ranges of the time necessitated multiple fuel stops in order to make long-haul journeys.

Once those necessities faded, both because of the build-out of airstrips worldwide (e.g. paved runways in Sri Lanka) and because of improving aircraft technology (no need to stop in Sumatra on the way from Java to Sri Lanka), there was no reason to go with an aircraft that couldn't land inland and had its aerodynamic properties constrained by the need for seaworthiness.