Technological rivalry galore!
Innovation and technology these days is very transient. It doesn’t take an awful while for something new to become obsolete. But things don’t necessarily have to be obsolete; sometimes just a smidgen of improvement is needed to set the pace of things. In light of Microsoft and Sony went about coming out with the Slim versions of their consoles, there seems to be no cause for them to not do the same for the current generation of consoles.
But why would we need a slimmer version of the Xbox One and the PS4 anyway? There are more than a handful of reasons.
The logic behind a slimmer version of consoles is to not only to flatter it out more, but to reduce component sizes or component placement or even changing the components used themselves. In any case, a lighter console using different materials is desired since toting it about would be much easier, no matter how trivial the travel distance.