> Or is straw simply a relic from when nonabsorbent insulation was
 > not available.

I'd say that is the case.  Straw, newspaper, wood chips, horse hair,
wool, and anything cheap, plentiful and at hand  was used as insulation
as late as the fifties, especially in do-it-yourself home construction
and rural areas.  The other options such as asbestos and rock wool were
expensive, hard to get some places and had limitations as well.

These materials have been deprecated for use in habitations for various
good reasons, including lack of consistency,  moisture retention,
settling, flammability, hazards to handlers and inhabitants, and
attraction of vermin, but the use in beekeeping continues.

Beekeepers have trouble measuring efficacy of wintering materials due to
the extreme variability of seasons, variations in the quality of bees
going into winter and a lack of precise measurement of success and
failure.

A difference of 50-100% in quality of wintering would not be apparent to
most beekeepers.  As long as the colonies mostly survive, significant
differences in strength and bee health are only marginally apparent.

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