Because you only split things out so far for parallelism, and that includes loaded tasks against actual ram.
If you want an active 8-core where everything gets used, you need a lot more ram than today, or apps written to exploit more parallelism. Probably both.
And unlike cores, sections of ram can't go to sleep, so that's going to be a power draw just for real estate.
BIGlittle has already been out and proven itself to a certain point, this is its next evolution. It's designed as an either-or deal to go fast or go efficient, on the fly.
The Tegra 3 quad core does this with a single little core (it's the quad with 5 cpus). Think of this like that, only the BIG gets a big boost going from the A9 to A15 architecture and the little gets a boost from one to quad to ease the transition from a single little core to multiple BIG cores.
Is this the best way? Is this a good way? Will it come through on promises? Will others just go on the big V-8 kinda engine? How much power in real life? How hot?
Your guess is as good as mine, I think we'll have to just see them.
But on paper, does it make sense? Yes, I think so.