Mainly because of better resolution and better colors. Hopefully they will get the lenses right too. And to hype things up: @PimaxUSA tried one a few months ago and he was super impressed: Apple Vision Pro's launch - #96 by PimaxUSA
That’s good enough for me 
Honestly I’m excited! I love my AVP but the moment I can get similar quality on a SteamVR compatible headset, I’m jumping ship!