Because the informed consumer expects a console to get upgraded mid-life and, thus, withholds purchase a few months later to wait for the slim model.
I can entertain the unlikely scenario that MS has a slim model unveiled at E3' 15 and, to prevent the upcoming social media and tech backlash from existing owners, gloriously presents everyone at the E3 conference their own slim model and is billed the hero.
Would X1 Slim's E3'15 unveiling be an unprecedented scenario by MS to attempt to win some of the millions of PS4 owners that have held back buying the original..?
I just don't believe this scenario... the Linkedin 'leak' refers to a 20nm revision that was planned along with the original 28nm and not some surprise 20nm model that has been secretly hatched recently because of X1's sales versus PS4's.
That this Linkedin would happen to coincide by 'karma' in some synchronicity with an unconventional secret unveiling of a new X1 slim, just doesn't intuitively hold any reality for me.
I'm happy to be wrong, I just suggest that speculation be more grounded in the likelihood that 20nm is just an internal chip revision of the existing model.