Hi Nikephorous - the HYW supplement hasn't gone. The original author dropped out so it has been handed over to another author.
From experience I can tell you that the supplements take quite a while to put together. Obviously there is the primary research required in the history, tactics and army composition. Each supplement contains several bespoke scenarios. These have to be play tested by others and amended before they are good enough to be included in the supplement. Once all the testing is done there is then a period of proofreading and layout before the supplement goes to print.
The supplements themselves are expensive to produce and with a relatively small player base they are by no means cash cows.
Running alongside the campaign supplements is the development of the main rules. Again this involves a considerable amount of time writing, testing, rewriting, testing again etc. As you know, a new edition of the rules is scheduled to be released next. As such it is important that any new supplements are developed with the new rules set in mind.
I cannot think of any other large battle ancients rules set (such as, War and Conquest, Clash of Empires, Hail ceasar, WRG etc) that gets anything near the support in terms of regular new products that Swordpoint does.
Here is a reminder of what has been produced so far:
Main Rule book
Dark Age Supplement
Chariot Wars Supplement
To The Ends of the Earth Supplement
Milites Mundi Rule Book
Genghis Khan Supplement
And here is a reminder of what is already scheduled for release:
Main Rule book version 2
Rise of Rome Supplement
Hundred Years War Supplement
I know that GB are always looking for willing authors and good quality material, so if there is a period that you are particularly interested in (such as the WOTR) then have a go at a supplement.