Given enough time anything can be picked apart to show its bad points. This is the whole idea behind selling a product. This is how its "proved" one thing is better than the other in the consumers eye. With a few adjustments poly thread may perform the same as cotton thread in the short term of a few dozen years. The manufacturers aren't really worried about the long haul unless its a company who's been around for hundreds of years and based their longevity around the product.
Poly is cheaper to make so more profit. Poly is renewable since it can be made from recycled "trash", Poly can be made into new and exciting types of threads. The stuff melts under high temps. Will it stand the test of time? Who Knows.
Cotton is an old product and more expensive since it must be grown and treated to be useable. Cotton can be changed in its coloring and is user friendly. Cotton doesn't melt and has been found in the pyramids. Its stood the test of time.