I'm sure Shady can give us a better explanation.
But I am thinking that for Dolby Atmos and Discrete Object-Based 3-D sound, we want more accurate and "focused" sound-field speakers, NOT a bunch of diffused sound everywhere, which is rather inaccurate.
That's why even before Atmos, I was never a fan of these diffused sound surround speakers - they are okay for creating ambience of non-discrete sound like the old Dolby PLIIx and DTS Neo.
But for discrete sound-field, I want more focused and accurate sound.