That's because your entire nation is the size of many states here, you have dozens of teams within a few miles of one another so a move is hardly an issue. Hell, you can move from one side of the island to the other and still have a do-able drive to watch a game in person. For me to drive to a game in California would take 3 days driving 15 hours a day. Philadelphia is relatively close to me, that's a six hour drive. So switching teams is more accepted here.
And this goes to the same point, you are comparing clubs that are so close together. You DO get plenty of news about most teams. Here it's about the local team mostly, others are secondary. Get yourself an NFL team and suddenly that changes. The talked about team is your local team, why would you be getting much talk about a team in California or Texas? Or hell, even in New York (or New ENGLAND?).
I understand the culture of sticking with your teams and that would certainly play some role in not getting as many fans as the NFL may like. But we have that here as well, people move and stay true to their teams. Take for example my daughter in San Diego, we visited during Football season and had to go with her to the local Bills Bar, it was insane, they were more into the game than any place out here, expats from Western New York now living in San Diego all tied to their team even more-so now that they are so far from home. And walking down the streets I saw pretty much every jersey you could imagine, and they all have local bars catering to those teams as well. Same thing here, we have plenty of other team bars who specialize in showing the Steelers or the Dolphins or Cowboys or packers, Bears, Eagles....
If London got herself a team, fans WOULD switch. Not all, not a chance! But enough and the switch would happen more and more over time, take a fan of say Seattle in London. Good the past few years, they will no doubt go through a bad time soon enough, a bad team gets less and less said about them, they are featured in fewer and fewer games that are televised, all while this fan in London is hearing all about their local team, they can go to a game or three, more and more people are talking about them, no,that Seahawks fan is going to change eventually, it will happen, I have no doubt about that in my mind! (or at least a large number of them will I should say) Plus, once the new team comes in, it's only natural to now follow the NEW team.