Because it's faster, lends itself to a simpler design and lets you spend more of your silicon budget on a more powerful GPU instead of embedded memory and functional blocks intended to keep the system from being bandwidth starved. Sony's also saving money elsewhere compared to the Durango. No HDMI passthrough, their camera should be cheaper to make than Kinect 2.0, and their APU may actually be cheaper to make since it doesn't have so much space devoted to ESRAM.
Keep in mind, Sony was originally targeting 4GB of GDDR5. That may have arguably still been a better trade-off performance wise than a segmented design like Durango, but if they found themselves in a position where they could give developers that 8GB of RAM they really wanted and still come in around the price point they targeted, that's a good decision. Like, if they wanted to sell these at $399 and with 4GB of GDDR5 they'd cost $330 each to build, but 8GB only puts that up to $390, the good will from developers and inability of MS to market 8GBs as an advantage is more valuable to the platform than pocketing the difference.