Because we are at a tech wall. I do not know what knowledge you have, but I highly suggest hanging out in the tech threads on here. I learned a lot more.
There is no way to have a generational leap with current tech yields/power consumption/cost, etc..
We will not see the availability of such tech for the mainstream $399 pricing until at least 2019/20 art the earliest. This is just an option to have more power, push 4K adoption which needs to happen now, NOT in 2020, help with VR, etc., for the same cost, in the same tech/ecosystem. So they are taking advantage of it.
It is optional, and will do nothing to effect current PS4 owners now. You do not want, do not upgrade. A slim was coming either way, they just decided to seize the opportunity to offer more power to help with their interest in 4K adoption.