If you don't think a movie is going to make a lot of money you don't spend a lot promoting it and you don't open it in 1100 1982 theaters. If it's not going to make a lot of money spending a bunch on advertising just means you lose more. Movies that are expected to gross quietly are released quietly. If you were going to make a mass appeal movie in 1981 you let Spielberg direct it, he'd just had Jaws, Close Encounters and Raiders (oh and 1941, which truly was a private movie that primarily existed so he could have fun with some of his favorite old time actors). The guy was 7 years off of inventing the summer block buster and here he was with another summer movie, that had it's opening weekend at 25 more theaters than Raiders. People probably didn't expect ET to make as much as it did, but there's no way it was a quiet little movie expected to make no money, quiet litte movies expected to make more money don't get opened at more theaters than Raiders, they get opened at fewer theaters than 1941 (325... all numbers from Mojo of course).
It appealed to people all over the world because that's what Spielberg is good at. Oh please, the Cannes audience is the most overrated bunch of egotists in the world, these are the clowns that worship Michael Moore, sophisticated might be on their press release but it isn't in their character traits.
Actually American Graffiti was what you're trying to cast ET as. There were a lot of complaints by the studio heads about the format of the movie (tracking stories that largely don't interact with each other) and the fact that it didn't have much of a point, and they generally thought 1973 was a little too early for 50s nostalgia.