Another reason is retail shelves are packed with nothing but AMD product. Gateway, HP -- the usual suspects sold at Best Buy, Office Max, Costco, Sam's Club etc are pushing AMD and Phenom x3 and x4 very hard. The ratio I saw was probably 5 AMD based PCs to 1 Intel. A bit shocking to see that AMD only has 18% of the desktop with that kind of presence on retail shelves.
That's not to discount the 780G -- AMD is definitely making a big deal out of the HD content decoding ability of the platform in pushing their X3 CPUs, as well they should.
I simply don't see the point of the x3 processor. If you don't need performance the cheapest single or dual core is the way to go -- AMD or Intel. If you need performance you either need as many cores as you can get your hands on *or* highest possible single core clocks. A low clocked 3 core cpu is neither fish nor fowl. Even if it comes close to the E7200 or x2 6400+ in a few highly parallel tasks it's going to fall well behind in most current applications. It'll draw more power at idle without offering compelling full blast performance to compensate. Just... pointless, except to move defective product which would otherwise be thrown away.
To be fair, I would like to add that if AMD decided to lower the prices even more, Phenom X3 may become a worthy alternative to Core 2 Duo E4000 and Pentium Dual Core.
Sums it up. The likely market is limited to ignorant consumers at current price levels.