Iron Man 2 opens this weekend, and it will not be shown in 3D. Why not? We talked to Hollywood insiders to get some answers. The first thing we learned: 3D is the current cash cow, responsible for the two biggest hits of 2010 thus far, Avatar and How to Train Your Dragon.

It's expensive to create 3D movies, but filmmakers can recoup their investment by charging more — a lot more — for movie tickets. There's a gold rush going on in 3D right now, with some studios such as DreamWorks Animation jumping in with plans to produce all their animated features in 3D from now on.

Dreamworks Animation studio head Jeffrey Katzenberg told us last month that this current version of 3D amounts to nothing short of a revolution. He said it's as important as the introduction of sound or color. Given all that, why isn't the summer's first megablockbuster, Iron Man 2, in 3D?

1. 3D costs too much.

Iron Man 2 was originally planned to be shot in 3D IMAX. Unfortunately, that idea didn't work out as director John Favreau had hoped. He said in a press conference last month that budget issues were one of the reasons why that didn't happen. What were the other reasons? Paramount, the studio responsible for IM2, wouldn't tell us, but Hollywood insiders say whenever there's a mystery in Tinseltown, it probably has something to do with money.

2. The film has too much live action.

According to veteran Hollywood visual effects compositor Ko Maruyama, Iron Man 2 contains so much live action that it wouldn't pay off as well as animated features. Says Maruyama, "Movies like Avatar and How to Train Your Dragon are created on computers with 3D software, which makes manipulating the stereo images much easier than it is for practical or live action."

3. It was a gamble.

Think of the historical context of the decision of whether to create Iron Man 2 in 3D or not. This was before the blockbuster success of Avatar, when no one knew if 3D would catch on with the public, or if it would even be profitable at all. In hindsight, it seems like an obvious decision to film a big-budget action flick in 3D, but that wasn't quite so clear-cut a year ago when principal photography for Iron Man 2 began.

4. Converting from 2D to 3D would've cost a ton — for poor results.

Now that we know audiences will line up for 3D movies, what about converting from 2D to 3D? There are issues: any time a foreground image overlaps background scenery — which is almost always — "it must be extracted," says Maruyama."Then the background elements need to be re-created, so they can be revealed cleanly when the stereoscopic parallax is visualized. That's going to run into some serious money."

Even after all that effort and expenditure, the results probably would have sucked. Just look at the overwhelmingly negative reviews for Clash of the Titans (here's a representative example), the poster boy for converting 2D to 3D. The consensus: Its 3D effects were downright lame compared to Avatar.

5. This movie didn't need 3D anyway.

Said a Hollywood insider who didn't want to be named, "IM2 is gonna be great because it has a good story with plenty of action — not because it's in 3D, and the producers know it. I don't think there is much in the telling of that story that would benefit from 3D."

The Bottom Line

We don't think there's much in the telling of any story that would benefit from 3D. Filmmakers want to milk this 3D cow as long as they can. This reincarnating 3D fad, which we find more of a distraction than an enhancement, could be short-lived. Hollywood wants to grab those premium-priced admission receipts while the getting's good. The production pipeline of Iron Man 2 might have been a bit early for its makers and investors to forsee the blockbuster profits 3D is currently yielding.

One thing's for sure, Iron Man 3 will be shot in 3D IMAX. The question is, by the time it hits theaters a couple of years from now, will viewers still be willing to pay double the price to watch it? We think not.