There's not any known reason why the bonfire amulet isn't able to be obtained as far as I'm aware. I did some research and couldn't find an official reply just this information seen:
It was implemented Jul 01 2009 - Major Game Update 8.50
Image and information are taken from my source: Jun 24 2009 - Further Changes...
"In addition to all this, there will also be a couple of new amulets. They basically work like the stone skin amulet, but instead of death damage protection they will grant you protection from different elements. Thus you will be able to withstand monsters dealing strong elemental damage much better."
And nothing is on Tibia Wiki aside from the information and it being "unobtainable" as far as we're awareYou see a bonfire amulet (protection physical +60%, fire +40%) that has 5 charges left. It can only be wielded properly by players of level 80 or higher. It weighs 7.00 oz. It engulfs you in flames - fire against fire.
Maybe it is obtainable and we just didn't find how to obtain it? Tibia Wiki Update 8.50 shows a lot of other interesting implementations and quests added during this time. Including the NPC A Bearded Woman for example, who is one of my favorites actually.