Because that was going to be MS's big marketing point for Nextbox power at E3!
I see so many of you guys confused on here why he even mentioned this when most people either don't care or aren't even interested in anything over 4k.
Well it's not a coincidence that Sony dropped this news the same day MS had their XO and dropped the SAD price.
Sony was killing 2 birds with one stone.
Simultaneously taking any wind out of the pitiful SAD box release and stealing MS's thunder from E319 that they were going to use for NextBox to hype it's power and the same group who are criticizing Mark Cerny for mentioning 8k now would have been EATING IT UP if MS was the first one to speak on it 100%.
The only reason Mark mentioned 8k WAS to be the first to speak on it, thus taking the wind out of any buzz MS might have had saying it during E3. Now that big marketing pitch MS had to hype NextBox will simply have gamers asking, "Ok, and what else?".