Far from a thread expert but I have a guess. The threads that are commonly used for tying flies have evolved from their sewing counterparts over the years. The compounding of the materials that form tying threads have stronger tinsel and torsion strengths. Of course common deduction would lead one to think that sewing thread should in fact have taken the same route seeings how added tenacity could not be a bad thing when repairing or creating an article of clothing.
In other words, I don't know