Different requirements drive different designs. Before WW2 seaplanes were common because of the lack of runways. After WW2 airports proliferated, and seaplanes couldn't keep up with technical advances due to the compromises involved in allowing them to land and take off from water. But that doesn't mean there aren't applications for aircraft with a flying boat's capabilities, it just means there isn't enough of a market in places like the US to support an industry. Even so, here in North America there are some 70 year-old WW2 Catalinas being used in aerial firefighting. China is a vast country which is prone to many kinds of natural disasters that could make airlifting in supplies difficult, so they may see potential applications we don't.
It's also interesting to note that seaplanes were highly useful in the pacific theater of WW2, and there hasn't been a protracted struggle for sea control *since* WW2. Also, China is a country with no operational aircraft carriers; aside from its training ship the Liaoning, it has a handful of amphibious assault ships that can carry a few helicopters. The US by contrast has ten supercarriers and nine amphibious assault ships that dwarf the aircraft carriers of WW2. The technology and expertise to run a carrier fleet like America's would take many years for China to develop. It's conceivable that the manufacturers imagine a military market for aircraft like this in the interim.