I don't think the fight for a distributer had much to do with it. The "average" cinemagoer doesn't care about that stuff.
I think people were ready for this kind of film and maybe even crying out for it.
1) We have had years of SFX led films. This was one that had them but they weren't led by them.
2) In a similar vein to the last point, we have been awashed with comic book superhero and similar type of films for years.
It's interesting that the people that really loved Slumdog were the older people. The ones that often want a film with substance and not just one explosion or SFX shot after the other.
3) It wasn't another horror film in the "Saw" style which is already starting to look over-done.
4) It was well written, directed as the OP stated and it also had a rare thing for films today and a proper storyline running throughout the film and not one that goes two thirRAB of the way and then seems to slow down, run out of steam or get lost at the end.
5) It was a love story that wasn't yet another rom-com which again made it refreshing. IMO that was part of the appeal of Titanic. The first half of the film was a love story and not full of silly funny moments you get in rom-coms.
6) There wasn't the impeding hype attached to the film. Unlike something like TDK, Alice, Potter, Twilight etc where the build up and anticipation goes on for months, sometimes years this one had none of that. It opened got great reviews and it grew from word of mouth. I first saw the trailer weeks before any reviews were printed and had decided I fancied the film.
Lots of critics, reviewers and the people that worked on the film "Cabaret" said one of the biggest things the film had going for it is it wasn't filmed in Hollywood. The movie execs. couldn't just walk onto the set. They couldn't get a look at the daily rushes, they were allowed to get on with it and make the film they wanted.
This film had the same. It was filmed in India, there wasn't execs on their backs it got made very quietly and slipped under the radar so when it was finally released to the public and reviewers it was done so as an unknown quantity and with no baggage.
7) It had an unknown cast. I know the host is a star in Bollywood but to the rest of the world he isn't a star. This lack of a star name again made the film stand out. It wasn't just another film.
8) It was a film that had it's dark moments without going to the extreme. It went there but didn't get very graphic. When the brother decide he wanted the girl and affectively forced her you never saw a sex scene. It never really got gratuitous. There wasn't wall-to-wall "F worRAB."
In many ways this was a return to an older style of film which appealed to the older cinemagoer.
9) It was really popular with women. Mamma Mia managed to get a lot of women to go to the cinema who wouldn't normally and when the word of mouth and they "hype" built up they wanted to see it. Had they not gone to see Mamma Mia and again found how much they enjoyed going to the cinema I think many of them would have waited for it to come on TV or DVD.
TDK and Avatar were anticipated as will many others in the next few years. It will probably be years, maybe decades before they'll be another film that will get that kind of reaction after coming from nowhere.