I won't claim to know for sure, but I think it is because there is more movement inside the case and that movement or positive pressure doesn't let the dust settle. At least that is all I can think of.
I edited my post with a response that seems to make sense. With a negative case pressure, you have a vacuum, and all those inlets that normally wouldn't move air start taking in air. Since the majority of high quality modern PC cases integrate some sort of filtration system at intake points, the problem would be greatly reduced, sure, you'll still want to clean your PC regularly, but the filters should take on most of the dust with a positive case pressure situation.
Also, the cooling is improved due to the nature of heats effect on air. It expands. The expansion causes the air to more readily move through the fans. Positive case pressure setups bring in more cool air, which is then expanded due to heat produced by your components, you have even more internal pressure. All this adds to the dust reduction properties of positive case pressure, as well.
Since the air is so tightly packed it more readily needs a method for escaping, moving through pores is a posibillity, but energy follows the path of least resistance, and well, hot air follows the same principal, doesn't it? Compressed air escapes through the compressed air can nozzle, not the bottom of the can which is sealed. The case I want has 1 exhaust fan, and 3 intake fans, with 2 fans inside helping to keep the momentum up after having been slowed down by the front fan systems filtration system.
PC-A70B by lian li. dual psu, simple looking, positive case pressure.
I use their picture because they seem to be the only one that carries the windowed version.
Testing in my armor showed that the fan placement that the lian li uses just past the hard drive cage improved my video card tempurature by almost 10 degrees celsius.