From my viewpoint, it is mainly a weight thing. When you're sitting on the saddle, you can exert varying amounts of power, which you adjust to whatever you can maintain. When you're pedaling out of the saddle, it more or less forces you to exert an amount of power proportional to your weight, so it's easier for a better rider (more power to weight).

It's kind of the difference between doing a bench press, using whatever weight you can handle, as compared to doing a pushup, which forces you into a particular lift.

Side note on technique: I was on a charity ride a while back, and was going up a hill at 11-12 mph. There was a fit young man on a CF bike riding out of the saddle, and I actually managed to pass him. When we crested the hill, he passed me up, and I didn't see him again. I think he was pedaling at too fast of a cadence, and also had a lot of bike-flop-side-to-side going on. He could probably have done better just pedaling from the saddle.