The problem is that Google is not just a developer, it's the platform owner, i.e. the company who has to sell the platform to manufacturers and carriers.
Carriers don't want features like Hangouts because they eat into their margins for crappy but expensive features like video-calls. If carriers don't want something, manufacturers (who have to sell to carriers) don't want it either, so Google (who have to sell to manufacturers) has to downplay it as well. Same goes for IM: carriers would prefer that you used their metered SMS services rather than ubiquitous GTalk or something equivalent.
Third-party developers like WhatsApp don't have to maintain a relationship with carriers, so they can push anything they want as hard as they want.
Carriers don't want features like Hangouts because they eat into their margins for crappy but expensive features like video-calls. If carriers don't want something, manufacturers (who have to sell to carriers) don't want it either, so Google (who have to sell to manufacturers) has to downplay it as well. Same goes for IM: carriers would prefer that you used their metered SMS services rather than ubiquitous GTalk or something equivalent.
Third-party developers like WhatsApp don't have to maintain a relationship with carriers, so they can push anything they want as hard as they want.