Gomez or Luton will correct me, but as I recall the way it was was this.
Channel 4 had started up as the fourth channel (there was no satellite) and it had a mandate to come up with "alternatives" to the mainstream channels. So it looked for alternative sports and came up with the NFL, which was a brilliant choice. This was the early 80s and, apart from a few people who'd lived in the U.S. and, of course, ex-pats, no one knew much about "American Football". Nevertheless, there was all this TV material that the NFL were only too delighted to sell on.
The format they came up with was to show a "game of the week" on Monday evenings taken from one of the Sunday games. They showed all the plays but cut the time-outs/half-time/two minute warnings, etc., etc, so the show ran for about 1 1/2 hours. The British newspapers weren't reporting the Sunday NFL games on Monday so you could watch the game without knowing the score -- and, of course, the game they picked would always be significant and/or exciting. It still seems to me the best way to watch football on TV. The only slight disadvantage was that, if a game looked like it was headed for a blow-out there would be a miracle come-back -- if it was 27-10 going into the fourth quarter you knew that the team with 10 would most likely score 2 TDs and a FG and it would head into over-time. Else they wouldn't be showing it.
Anyway, that was an amazingly successful format which developed a surprisingly large public for football very quickly. But of course the teams that were shown mostly were the teams that were involved in the most crucial and exciting games. I don't think I can remember EVER seeing a Cards game and New England (until the 85 season) were pretty rare (I'd lived in NE for a season, so I didn't pick a team off the screen; but I did tend to root for SF and San Diego because I liked the way they played and, at least, we got to see them in some games).
So you shouldn't blame the UK public for choosing whom they did. It's just a shame that the football coverage here is now so cr*ppy that I doubt many new fans are being added.