There are plenty of us that do care about it. Sure, they didn't have to do it, they still would have had plenty of users - but they chose to do it.
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.
To have a good grasp of what matters to the public, remember that Apple once did a whole ad campaing over the fact that the new iPhone could copy/paste. We mocked it. And the joke is on us, cause it worked briantly. This is what people cares about, and more over, this is what people knows about.