It's because of how old it is. The fact it was filmed in 1941 and one of the first "Blockbuster" films to be widely received. I feel the same way about Casablanca. It's a good movie with a solid plot, strong characterization and was released early on during WWII but it's really not all that fucking all mighty untouchable as everyone claims.
I know it's an unfair label but these older films get the praise they do because they're the first of their caliber.
However, Mr. Smith Goes to Washington and The Grapes of Wrath deserve all the praise they get. Those movies are excellent.