Since the WoC a Chance Encounter farm was introduced to the game, most of the value heavily dropped, especially the last few years. You can with minimal investment farm that quest over and over again, only requirement being to have a good hero team with the best armor/weapons/insignias. I as a solo player have pulled at least 20a worth of shields/staves/caster items from there. So you can only imagine what the bots running that place are pouring into the economy.
Kaineng Center has become Kaineng Factory, running 24/7. Dropping Bo/platinum/dragons non stop. So I believe it has become a preference thing between these items.
From my experience, (active speedclearer) Bo's are the introduction to expensive OS items for people that don't pay too much attention to OS stuff. You can't get it cheap as an inscribable version (you can with Platinum, Zodiac, Celestials), and a lot of people like the clean look of it
For me personally, it has always been:
Forbidden
Platinum
Bo
the rest.
I rarely see Forbidden staves sell, and when they do, for some reason, they don't sell too good either. Or else I'd farm them all day long since they drop from one of my favourite chestruns.
Cockatrice and Outcast staves are also botted a lot, so there's a lot of them selling and they frequently enter the market.
Why Zodiacs have become so rare is probably because Urgoz and Deep aren't really botted to the same extent. There's a chest run to do on each, but it's not consistent enough for it to be worth it. You more than often get a run without a chest in it, sometimes can't even reach it. So Zodiacs, or anything that exclusively drops from Deep/Urgoz (exalted aegis, Zodiac shields/weapons, wayward wands etc) are mostly coming from player runs, which is getting increasingly rare to come by. Gathering 12 people that want's to do more than 2 consecutive runs is hard and the only consistent guild I know that actually does not run skipway (kill a lot, loot stuff) is [Lgit]. There is quite a few big groups of enemies that are spiked, leaving a lot to loot. You really have to try a Deep non skipway run with favor up.
Urgoz is mostly skipway so drops are minimal there but I believe Zraw is one of the guilds consistently running Urgoz.
As an example, I got an insanely lucky Zodiac staff drop last year, Q9 20/20 Dom. While searching for a reference price, the only thing I could find was a post from 2 years ago and that was a Q10 20/20. They rarely enter the market and there's quite a few people looking for them.
TLDR; Bots made a lot of them easily obtainable, leaving it to the community to pick the favourite skins. The ones the bots don't touch, became rarer.