Everyone dying is a staple of the zombie genre, but mostly it's just a staple because everyone else did it, not because it's a particularly trope.

Well ... yes and no, I think; it's a trope of horror in general, and zombie movies are a subset of horror. The whole focus on death and dying and "any of US could become one of THEM!" in zombie movies kinda lends itself to killing off characters.

Having said that, no, there's no specific reason why most of the cast has to die.

I haven't seen Land of the Dead -- I'm not generally a big zombie movie buff -- but I remember being intrigued by the promos.