It was a 1946 film and it was beloved by film buffs in the 60s and 70s. They passed along their enthuasiasm for it to the general public. Same with John ford's The Searchers which was regarded as 'just another John Ford/John Wayne movie'. Anyway, IAWL is a masterpiece. It's a mix of film noir and a whole bunch of other things.
IAWL is a good movie, but not a "masterpiece" and in terms of how much it's overplayed and built up as the greatest Xmas film (it is NOT a Xmas film, it is primarily a biography!), it is overrated.
Beloved of "film buffs" probably tells me all I need to know. ;-) As a child in the '70s I never once had even heard of it, and I watched old movies alot on plain TV (no VCRs in those days, you know). Apparently the film buffs didn't get their way until the late '80s.
Again, it IS a very nice film and 1 I like to watch, but it's just overrated (<- means "rated higher than it should be", not "bad").