Are you talking about just Sony consoles? Because otherwise your observation is false: Microsoft Xbox One, for example, got an upgrade from DVD to BluRay as sam

Are you talking about just Sony consoles? Because otherwise your observation is false: Microsoft Xbox One, for example, got an upgrade from DVD to BluRay as same as did Wii. And in previous generations both did not get any upgrades. The reason Sony upgrades media on its consoles is that Sony is a holding that also holds companies developing the said media and companies distributing movies/music on the same media. It's a cross-promotion.