Because the sales of Pro and X demonstrably evidence a mass market for more powerful hardware?
Sony's three best selling consoles are primarily, affordable and game focussed. Microsoft's best selling console was affordable and game focussed. These are not the only factors that matter but they do seem quite important for a game console
Exaclty what i was thinking too, the worlds most successfull console is the PS2, that thing launched at $299, had nice price-drops quickly too. Even at that 'low price'/relative weak hardware, it managed to impress graphically, people will never forget MGS2, tekken tag, GoW, GT3, just to name a few. Not only did it impress graphically but games wise i feel no other console has ever topped the PS2's library. It was in production for 13 years.
Dont know why some want all out on powa, massive tflops and ram whatever, but if theres one thing i have learned over the years, here on B3D too, is that its Software that makes a platform, much less so the hardware. Personally think the PS4 is too expensive both now and at launch, a cheaper price will reach out to a bigger audience, a bigger audience means more software sales and thus better games.
Atleast the PS4 was in the right direction after the PS3, maybe PS5 will be like PS2?