There's a small market for it.
Computers don't cost that much to run for typical use scenarios to begin with. In a business setting, they're in use for only 40 out of 168 hours per week, otherwise they're idle or powered off. During use, they're generally only running browsers and productivity/business applications, not sipping much power.
Let's say their green CPUs offer a 20% lower power consumption benefit. A company with 1000 employees and 1000 computers currently spends $30/year per computer per year in operating costs. That's $30,000/year in electricity costs, sure.. but then a 20% savings brings the bill down to $24,000, only saving them $6000/year. But even that's not right because it's only a 20% reduction on the processor.. other components aren't "green". So maybe it's only saving $4000/year. That's a company with 1000 employees, they're probably already making hundreds of millions if not billions per year, $4k is insignificant.
For a typical consumer, yearly power consumption of a home computer is already low enough to be considered more or less insignificant. Power users see "green" CPUs as low powered/gimped processors.
So who'd buying these then? Maybe environmentally conscious startup businesses? End users who are obsessed with everything being green? That's limiting market reach a lot. Then you have to consider business partners like Dell and HP. Do they want "green" processors advertised in their systems? It limits who wants to buy them.
AMD is focusing power savings where it matters most and makes the most business sense. Enterprise, where power costs are astronomical, and laptops, where battery life is important.