I have no experience or expertise on the subject, but I think that the irons probably 'burn out' for the same reasons as Light Bulbs do.

Ie. Oxidation on the heating element, which reduces its CSA --> Increases its resistance --> increases the temperature--> speeds up the oxidation -->-->--> you get the idea.
One big vicious circle.

Possibly (more like defiantly) the more expensive irons use higher quality materials. As opposed to the cheep ones that are made from ... I dunno, Fuse wire??