There is a simple reason why bounties after Queen were not bothered to be revealed.
The reason is to allow Oda to write while ignoring the whole bounties = PL misconception/perception. I know that's generally not true, but you cannot deny most fans see it that way, and the story also strongly implies it as so.
Every time a bounty was revealed, it was to hype up the villain/opponent that SHs were going to fight or deal with.
If you were to reveal P1 and Ulti's bounties before they fought Luffy then most fans would just be like "How are these 290 mil/400 mil bounty fighters giving trouble to a 1.5 bil bounty fighter?!" Oda recognizes this problem in his storytelling.
See what I mean? By getting rid of bounties and only mention them in passing, Oda is allowed more freedom in powerscaling whatever he wants to be. He still brings in general bounties like Marco's "Facing two 1 bil bounty fighters is tough!" just to bring some sense of scaling into the story via bounties but nothing concrete, which is how most powerscaling in Shonen works.
He revealed Queen's bounty because 1) bring some sense of tension to Udon while Luffy was severely nerfed, 2) to hype BM's monstrous power even when she is as nerfed as Luffy, that being a Yonko means two-shotting a 1.3 bil bounty is nothing for them.
And I'm also guessing that Oda wanted to deliberately leave out Marco and King's bounties because 1) he already revealed Yonko bounties so it would be backwards to hype inferior characters, 2) he did not want to make fellow YC1 Katakuri seem less inferior than both of them through some crap PL system that he didn't think very thoroughly.
So no, it was not because King wasn't important. King has more lore in his history than any other SH besides Robin. It was because Oda placed too much emphasis on bounties as PL scouters and he wanted to take that out of the equation to bring us match-ups while thinking of the characters' abilities and true strengths, not their bounties.
