Well, for a number of reasons.
1. It was not made with artistic intentions but as mainstream entertainment with popular stars and, also, as an anti-German film in the early stages of WW II.
2. The supporting character actors, all, such as Peter Lorre, Sydney Greenstreet, Dooley Wilson, and the German officer, Conrad Veidt, are terrific, even the minor characters are perfect.
3. The dialogues are among the wittiest ever in an American film with one unforgettable one-liner after another. ex: "round up the usual suspects" -- "of all the gin joints in the world she has to walk into mine", "that's my least vulnerable spot (my heart), You want my advice? ~ Go back to Bulgaria...etcetera.
4. The pacing is perfect.
5. The Marseillaise scene is absolutely stirring, no matter how many times you see it
6. The twist at the end is perfectly shocking but acceptable and inevitable -- it turns the picture into a Masterpiece and raises it above merely pure entertainment -- but it is purely entertaining nevertheless, from beginning to end.
7. There is nothing pretentious about it -- it just flows.
8. A perfect mix of romance, suspense, threat, fidelity, betrayal, humor, patriotism, and everything else you can think of, running the gamut of human emotions.
9. Director Michael Curtiz, originally of Hungary, was one of the best contract directors in Hollywood. in spite of his heavy Hungarian accent which actors could barely understand. He made films in nearly every genre, adventure, romance, westerns, and even one musical.
You could hand him any kind of script and he would make the best film possible movie out of it with the given material because he was a Master of the medium.
Casablanca is an object lesson
Study it carefully for the editing, the camera placement, and especially the duration of scenes. He never lets a scene run on too long, or cuts it off too short.
He was a true master of the medium. Every time I watch Casablanca I see new things... or get paroxysms of glee over things I've seen before. From the very opening where we see a huge world globe turning slowly and coming to rest on ...... Casablanca!
The casting of european actor Paul Henreid was the only slightly weak link in the picture, but even that Curtiz handled as well as possible. He did not have total control over the casting.
By Alex Deleon for filmfestivals.com