Because one is a massively successful, if rightfully derided,title and the other is a huge event in gaming. Sony are absolutely right to beat their chest about both of these, especially since Bungie's past could suggest that Destiny would be a MS exclusive.
If you think anyone
was sat during the PS4 reveal hoping for a Diablo3 announcement then you're deluding yourself.
Well, I found it a more interesting announcement than the "share" button at least and I'm sure there's some people who want to try it out who don't have the PC to run it (the offline mode could even tempt those who have played it on the PC). So maybe no one was hoping for it, waving their little Blizzard flag in anticipation, but I don't think it was unwelcome news.
I'm still not entirely sure what point your trying to make, is awareness of new titles a bad thing all of a sudden?