Because as i said the games there are played there 3 times a year are sold out 10 months in advance - the 3 for next year they expect will be sold out in Jan - the England football team has no relevance.
The foreign import is proving to be massively popular throughout the country.
But the point you are missing if there is a UK franchise in the NFL there will be 8 regular season games. That's an awful lot of cash to shell out for families especially those not adjacent to Wembley and the interest wouldn't be there season on season. It's fine for one off games where fans can pick and choose which one they want to go to