Any question answered...

Why did seatposts and headsets stop being part of groupsets?

Probably because OEMs stopped buying them when they realised that most people didn't care about cheaper Taiwanese substitutions. Also, frame materials changed, so what was once a case of having 27.2 and 26.8 to cover most steel frames suddenly became 20 different seat post sizes to suit oversized steel, aluminium and latterly carbon frames. That's fine if you're selling thousands at a time to an OEM, but a PITA for a shop trying to stock three or four grades each of Campag and Shimano in every size your customers might ask for, so the after-market upgrades went out of fashion too. Seat posts are pretty easy to make, so they were one of the things which the boutique manufacturers jumped on back in the purple anodised days, and it was hard for Campag and Shimano to come up with a product which they could stand behind which would compete on paper with something knocked up in a shed by some bloke with nothing to lose if it proved failure-prone.

On headsets, Dia Compe came up with a better way to skin the cat, and Shimano refused to pay for a licence (they are always deeply reluctant to use pay for other people's IP), which basically killed Shimano as a mainstream headset option, and I doubt that they were too bothered by the loss.