That's the simpler explanation, but Occam's Razor is a thin reed to lean on, because simpler doesn't equate to correctness. Let's inspect: "the US market doesn't give jack shit". But how would we know? The media hasn't even tried.
I imagine this would piss me off if I were a Rachael-Maddow-watching soccer mom. Show anchors are hired for their ability to work up their viewers. Taking pics of me in my underwear is a softball. Notice rasz_pl's comment below -- any producer worth a damn sees the potential in this scandal. So why is Brian Williams not leading off with it on the Nightly News as he did with the snoopware-webcam story from over a year ago? Who the fuck can tell me that? Open your fucking eyes man, and start asking your own questions.
As for "this only pertains to UK citizens" - this program wasn't limited to the UK. In many ways, the UK spying on the US is more sensational. Furthermore, the NSA and GCHQ have blurred the lines between jurisdictions. As for "only netizens would care" -- most people use the Internet. Yahoo is an everyman portal. This is an egregious invasion of privacy. Who cares if you're a netizen on this story? The story's content is not to blame either; general viewership can be enraged about esoteric things too (a great ex. the "bridge to nowhere" - just think about it).
The US would care if the baby-boomer/mainstream media tried to cover it, at all.