Horseshit nationalism apart, there are two really big reasons not to do this:
(1) Competitive balance. I can't see any way that a team based a seven-hour flight away from the nearest U.S. teams could compete on equal terms
(2) The legal-financial framework. The NFL is a number of businesses operating as one (draft, revenue sharing) and has a complicated (but very beneficial) relationship with the regulatory authorities (anti-trust exemption). I'm no lawyer, but the number of complications that would come into play if you extend the business outside the U.S. are pretty staggering. And you can't assume that politicians in the U.K. would bend over backwards to help in the way that they do in the U.S.
So why do they want to do this so badly? Even if you think (like me) that Goodell is a $40 million water-boy, he's a spokesman for a bunch of very rich people, some of whom made the money themselves and can be assumed to be very, very smart (so, not Woody Johnson).
I think the answer isn't increased franchise revenue (after all, adding more franchises just dilutes things with revenue sharing) but the desire for an international profile to be associated with international sponsors.
The top soccer teams are brands that are world-wide. Their exhibition games in the Far East or anywhere else are packed out. Sponsors know that and are prepared to pay for it.
Mr Kraft and his fellow owners want a list like this:
http://www.manutd.com/en/Club/Sponsors.aspx
Not Bob's Discount Furniture and Putnam Investments
EDIT: How weird is it that Manchester United have an Official Global Noodle Partner?