Cause that's really all it's got. And $100 less.
True the differences in graphics are going to be really minor at best. Remember the Xbox was over twice as powerful as the PS2 and it didn't really matter.
Xbox one has so many other enhancing features that will make gaming and games more interesting. Then there is all the others stuff it can do, and more stuff it will be able to do in the future.
The PS4 should be a fine gaming system, but some are so worried about it 'loosing' they just can't help themselves.