I'm not saying that the current superhero era in cinema started with the 1978 Superman, although you could argue it had some prolonged influence on the genre, but specifically that the Raimi and Nolan trilogies started it.

Why was Iron Man made, over a movie about a famous hero? It was made because Marvel recognized the success of the above two trilogies but had lost the film rights to most of its popular characters.

Marvel wasn't in any position to be taking huge risks. They had barely scraped by from their financial woes a decade earlier, and if Spider-Man and Batman Begins hadn't come along first, I personally doubt that Marvel would've put out The Incredible Hulk or Iron Man when they did.

There's also certainly an argument to be made about improving visual effects making more viable the creation of superhero movies and the superhero feats they entail.