I don't really get why there isn't BC this time. All the upgrades are just moare bigger cpu and ram and crap.
But they're completely different architectures.
The only reason console games are able to look as good as they are with as underpowered as the systems are (try running Mass Effect 3 on a PC with 512mb ram and let me know how it goes) is because the games are coded specifically for the architecture of those consoles. It's a major advantage consoles have over PCs. When they develop a game they know exactly what system everyone is going to be playing it on. That's why they're able to do so much more with so much less.
Both Sony and MS switched to the x86 architecture this gen, so that eliminates that advantage on backwards compatible games. That means the only option for true BC (not cloud streamed) is to either put 360/PS3 hardware inside the new systems (which would raise the price of each by about $100, make them physically larger, and add additional heat) or try and emulate 360/PS3 games.
Neither the PS4 or XB1 are nearly powerful enough to emulate 360/PS3 games. Even high end PCs can't do that yet.