I just see this as a relatively new phenomenon and it's kind of baffling because I don't see what, exactly, caused it.
I can't think of a first movie that blew its load and made its own sequels ineffectual.
Burton came smashing out of the gate with the Joker, Batman's biggest and most popular nemesis. It didn't ruin the series, and he wasn't "too much for a rookie Batman to handle." And it became one of the most successful movies of all time.
I can't help but think that, nowadays, people would be decrying that decision and wondering why Burton didn't open the series with Clayface and then "work up to the Joker."