The argument is that knowledge and understanding of encryption is limited to a very small audience even amongst "tech savvy" users.
Even if you only take software developers the vast majority of them does not understands or "cares" about encryption, it's just the hard truth of the matter.
WhatsApp did it because they could do it in a way that was beneficial to them coupled quite likely with internal ideological reasons to roll it out.
It's a positive spin that can be spun in the tech and non-tech media and could be coupled to improve their reputation as long as it's being run with the overall privacy / encryption debate stories.
It also future proofs their platform that when they do come under direct fire they don't have to be seen as reactive.
WhatsApp did it because they could do it in a way that was beneficial to them coupled quite likely with internal ideological reasons to roll it out. It's a positive spin that can be spun in the tech and non-tech media and could be coupled to improve their reputation as long as it's being run with the overall privacy / encryption debate stories. It also future proofs their platform that when they do come under direct fire they don't have to be seen as reactive.