As others pointed out, with many finishing invader, they want good talis in it - or finishing vanq/opp and wanting good talis, or rr61 sc weapons, or bloodlord pve weapons. Demand increases, supply remains slow or diminished (ppl not bothering with ch22 like they did last year).
Then if you have AH with barely 30-50 purple 24s talis left, and 5-10 people with enough money [5-50k] realize its good opportunity to make extra gold by clearing the AH and reselling at higher prices, good for them, that's a good way to make extra money.
I think in past you could dump your extra officer medals to buy Anni gear, salvage it, and have chance at lvl 200 mats for extra talis - apparently this was nerfed so ppl might as well dump their officer medals down the toilet after first 10k because you probably already loaded with pots. Salvaging invader or vanq is not as easy as the old Anni salvaging - and not sure if conq gear can be turned into lvl 200 purple mats.
As for explaining why purple armor talis are rare, from having run 15-20 BS runs, I have gained ONE lvl 200 purple armor fragment, few 2-4 other frags (no one cares for willpower talis anyway or toughness). Instanced bosses are known for providing lvl 200 green trash mats, with extreme luck you might get a blue lvl 200 fragment. Maybe 1 mob in 100 drops blue fragment, and maybe 1 in 1000 might drop a purple fragment. And then people wonder why no one is producing enough talismans to fill everyone and AFK-alts invader gear with talismans.
