I think it's because most birders are simple folk who can't get their
tongues around the scientific avifaunal nomenclature.
The birders I get around with though have progressed past the stage wherein
a Common English Name ( as advocated by Birds Aust) is adequate, because we
now identify birds not just by species but also by subspecies names , and
one has to know them and be able to pronounce them to be communicate
effectively at that level, and as you do with plants, one tends to use the
subspecies or race name when calling birds, often omitting the species
and/or nominate names.
Twitching at the subspecies level demands knowledge that, for instance,
Schodde breaks the dear old Grey Shrike-thrush into nearly a dozen subs.,
and it provides a far wider and more interesting range of birds to identify,
both in terms of their physical differences and their
So, I think it's largely a matter of with whom one associates and
communicates and where one travels in their birding quests.
Tony the Mex.