Well most importantly, its because Xbox 1 itself is so anemic that the upgrade is far more drastic than the PS4's upgrade to Pro, which generally stretches the suspension of belief for some that games will be limited to XB1 levels where any other console in that realm of upgrade would be considered close to a new generational upgrade.
The Scorpio's GPU alone looks to be close to 6x more processing power than the XB1's GPU, not counting the 4 years worth of GPU architecture advancements from XB1 launch to Scorpio launch.
A potential Zen CPU enough would be worth a new gen console IMO, but 6tflops and 12GB ram at 320GB/s(the rumored basic specs) is pretty good too as an upgrade as well.
Compare that to PS4 to Pro's 1.84 tflop to 4.2(exactly 2x GPU) 30% CPU and 30% Bandwidth, and the differences are much more reserved from PS4 to Pro.
Of course, that makes sense with Sony's strategy. If Pro is a midgen upgrade, then the console they actually put effort into next(IE PS5) will be significantly stronger than base PS4,but with the addition of having its own library taking full advantage of the platform
My guess for PS5 has always been about 10tflops, 16GB RAM, 500+GB/s, and Zen.
Now, all that having been said, a lot of those rumors for Scorpio are still unconfirmed, especially the Zen CPU which has already been claimed by AMD itself to not be ready for console style integration until 2018, past the point Scorpio will be out in the wild. Having the same Jaguar CPU would essentially handicap the Scorpio significantly.