Last ? first: Taffeta is a slippery, flat, smooth, square-woven, lightweight fabric. Makes it easier to get your arm through a damp sleeve.
Next: Polyester is more resistant to UV, petrochemicals, acids, etc. than nylon. It stretches less. Nylon's stretch protects it from shock loads and helps account for its 2X strength compared to polyester...when new, not after lots of sun, DEET, and spilled lemon drink powder. Even coated Nylon should be treated with DWR to reduce water absorption, sag, stretch, etc.
Polyester is vastly superior to nylon for tent flies and tarps because it does not absorb as much water and does not stretch and sag when wet. Some makers use it. I have a poly tent from 1979. Still good.
Nylon is what the mills make because it is what the public has come to expect. Or maybe it's the other way around. Lack of gumption, imagination, vision, whatever, may also play a part.
A mill with its head at the right end would produce Spectra/Dyneema reinforced polyester fabrics suitable to replace many or most of the current nylon offerings. Spectra cannot be successfully dyed, but polyester can. The polyester component will also accept polyurethane and silicone coatings.
Imagine a down-proof, uncoated fabric equal in strength to 1.1 oz nylon but weighing 0.3 to 0.5 oz. Or the same fabric with 0.2 or 0.3 oz coating (typical) that would work for lightweight tarps. flies, pack covers, UL raingear and UL packs. Or an intimate blend of Spectra (not a grid-stop) spun into polyester Cordura yarns to produce strong, abrasion resistant pack cloths ranging from 1.3 to 3 ounces.
You noted the use of polyester thread on nylon items. It's a compromise. Remember, nylon stretches and polyester does not. Ideally you would use a stretchy thread on stretchy fabric or on low-stretch fabric. A good jerk on nylon tarp hem, stitched with polyester, will break the thread wherever it is too tight. On the other hand, some home machines don't handle nylon thread too well (because of its stretch) and in most cases, polyester thread will give neater results. For many applications, nylon thread is overkill. I use polyester thread on day packs, tarps, hammocks, UL packs and all stitching on silnylon. On long seams and tarp hems, I use a shallow zigzag. That keeps the nylon from snapping the thread.