"Niche conservatism is the tendency of species to retain ancestral ecological characteristics." (PDF)
But why do species have this tendency, or more particularly, why do ecological traits not change more rapidly than they do as a lineage evolves? Wiens and Graham state that, "We refer to niche conservatism as a process, although it may be caused by more than one factor at the population level," but don't say what those factors may be, or provide a reference to someone who does.
So what causes niche conservatism:
Two basic mechanisms occur to me. The first is that new traits simply don't arise, and therefore can't be selected for or against. Why don't we have rats that can eat iron? There is lots of elemental iron around these days, and it is very high in energy. Surely a rat that could eat iron and extract all that energy would do very well, and make lots more iron-eating rats. But that trait has not occurred (and is not likely to do so) and as such there is no way for natural selection to favor the iron-eating rats. But for traits like sexual mass dimorphism, this type of constraint seems unlikely. We know that closely related species sometimes have very different mass dimorphisms (compare gorillas to humans). But closely related species usually don't have wildly different mass dimorphisms, so there is conservatism in the trait even though it is pliable.
The second possibility that occurs is that niche conservatism comes about simply because a trait works well with the way a population lives, and the other traits is has, and therefore some form of selection acts to keep the trait where it is. The trait is conserved because it is functional. Even if some other combination of traits would be more advantageous, that combination would require so many modifications that you can't get there from here. The population would have to go to far 'downhill' on the fitness landscape to get to that taller hill across the way. So selection, which tends to push the population 'uphill,' keeps it on the little local peak it is on. By keeping the population where it is, selection causes niche conservatism.
I'll call it a hypothesis. First I need to find out who has already said almost the same thing.