Cost reduction. You take something that's quite obviously geared towards the high end audio market, remove it (there's still the Neo for those folks), and save $2 off the final cost. Doesn't seem like much, but you figure they probably plan to sell another 30 million of these at least, that's 60 million bucks.
Personally I think the slim is more of a lateral move to the standard PS4, 5mhz wifi is nice if you need it, but the space savings is bogus, things like this the footprint is what really matters. "If this PS4 was only 3/4 of an inch shorter I could cram something on top of it in my TV stand" said no one with a brain, ever.

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