Why did AMD make the 8xx series chipsets when the 7xx chipsets seemed to do fine? Well, among other minor things, it really added proper support for AMD TurboCore 1.0 technology. From speaking with Asus engineers who managed to make the TurboCore "work" on my 790FX board, they said it was more of a hack, one that even after two BIOS revisions, I still keep turned off because it is buggy.
The 990FX chipset should include HTT3.1, PCI-Express 3.0 (I think), and TurboCore 2.0 support. I suspect the latter to be the most relevant in AMDs decision to exclude AM3 boards, as I don't believe per-core voltage modification is supported (and if it is, likely not how the Zambezi chips require it). In the past, AMD chips never had much of a performance difference from HTT 1.0 -> 2.0 and so on, but maybe it will be more important for Zambezi?
Unfortunately, I don't see native USB3 support coming (I think the Llano chipset will be the only one to see that), but I do foresee many new motherboards being released supporting the UEFI standard over the BIOS.
The question I have for some of you all is that if the reason is money, why would Asus be making Zambezi compatible BIOSes for current motherboards? A stronger argument is why would they announce them now, potentially hurting future motherboard sales, instead of just silently releasing updates once they've become satisfied by AM3+ motherboard profits?