Why Zoroastrians refuse to bury or burn the bodies of the dead?

I'm not entirely sure on this one, but i believe they respect the 4 elements. Water, Earth, Fire, Air. And to bury or burn the dead, is to desecrate the Earth and Fire respectively.

I believe they traditionally would have a stone lined pit with platforms circling it, and they would leave the body on the platform. The birds and other animals would come and pick the bones clean in a matter of days, and then the bones dropped into the pit. That way, nothing was desecrated.

See the link. It is not entirely true that Zoroastrians do not bury or burn their dead. Some follow these practices. In some groups, particularly India where it is not against the law, some Zoroastrians leave corpses out (ritual exposure) as an alternate method of clean disposal. The corpse is an agent of decay and its disposal can't pollute 'good' creation. This involves allowing the body to be consumed by scavenger birds. But in some places drugs administered to animals have seriously dessimated the population of vultures, and ritual exposure does not happen as it otherwise would. In those places Zoroastrians cremate their dead, or bury the bodies in graves lined with lime mortar.