For the same reason why machine guns and Boeing 727s don't appear on the equipment lists in the PHB, maybe. Maybe wikipedia can help explain the concept:
http://en.wikipedia.org/wiki/Anachronism
I don't think anachronisms as such are much of a problem to accept if they're consistently and pervasively drawing from more than one tech level, except that in Eberron the exceptions are highly selective, and therefore IMO incongruous.
If everyone in Eberron was right up there with the dual tech levels, with dwarven steamrollers, halfling biplanes, black powder weaponry as common as swords etc. like in Arcanum, I think that would have been a much better way to handle it. But they couldn't because the setting had to cleave to generic D&D assumptions as well in order to meet WOTC needs. As it is, because they've gone "implied setting tech level, but with the odd incongruous quirk" the anachronisms that are there stick out like sore thumbs IMO.