It's not really possible to know exactly why they closed NFL Europe. It was losing money, but it was always losing money - a few tens of millions of (US) Dollars per season ($30MM by the end). Why pick 2007?
It's possible, of course, that it was simply money - certainly $30MM a year is not nothing. It's also likely, though, that owners had started to think about developmental leagues a bit differently (which is what NFL Europe was, basically). In this article in the Guardian, Cowboys EVP Stephen Jones said:
“We’ve reached a point where we really should be looking more at a developmental league, and I really expect that to begin here in the next year or so,” Dallas Cowboys executive vice president Stephen Jones, a member of the league’s competition committee, told CBS Sports. “We lost a lot of money on NFL Europe, but there were a lot of things the league did well and for all of those reasons – coaches, officiating, players, quarterbacks in particular – it’s something we really do need to be looking at and studying. The time might be right to do it.”
They never did start a developmental league (at least, not as of 2021); my suspicion is simply that it's hard to do football "minor leagues" as you need so many players, and it's so physically demanding and dangerous. Perhaps whomever was thinking of starting a new league lost influence - or nobody was, and Mr. Jones was just blowing hot air.
You can read some more about the league and its failures here.
