The reason your hero is unaware of the details pertaining to the Worldstone’s destruction is because the Barbarian you play in Diablo III is not the same hero from Diablo II. Also, twenty years have passed between the events of Diablo II: Lord of Destruction and the start of Diablo III, so much of what transpired is not common knowledge.
I already typed a long response to this, but then it just disappeared when I clicked submit, this seems to happen a lot on these forums.. I wonder if it is my browser or this website...
I'll try to remember what I said I retype the main points:
*It is impossible for our Diablo 3 Barbarian hero to not know what occurred on Mt. Arreat unless (a) he has selective amnesia, (b) the Barbarian hero from Diablo 2 had selective amnesia/post traumatic stress disorder, (c) the Barbarian hero from Diablo 2 refused to share his story and never spoke of what occurred or (d) the Barbarian hero failed to travel through the portal and died in the explosion (or he survived but died almost immediately after the destruction of Mt. Arreat).
I have read some of the lore, but I admit I haven't read it all and I don't know if there are novels or not or other sources of information that I haven't read. But based upon what I do know, I'm going to analyze my own theories above in reverse order.
(d) The Barbarian hero who killed (or helped to kill) Baal died. This is very likely. This could have occurred during the big battle with Baal, but I remember reading or seeing something that implied the heroes celebrated their victory afterwards. I wish I remembered where I saw this.. anyway.. from what I know, we're led to believe the Barbarian hero from Diablo 2:LoD survived as he escaped through the portal. We're looking at this from the perspective that the player succeeds, so the character succeeds. If anyone knows of a book or piece of lore that contradicts this, please show me where I'm wrong.
(c) While the Barbarians from Diablo 2 and 3 are both men of few words, they do talk. Unless the hero had amnesia, I doubt he would hide the truth from his brothers and sisters..his fellow Barbarian folk. In fact, after the loss of Mt. Arreat.. something that they had guarded intently for generations.. so much so that even the ghosts of their Ancients guarded the mountain.. I would think that the barbarian folk would try to band together and share anything they knew before eventually going off on their own to explore the world now open to them. Likely, many barbarians would look to the hero from Diablo 2 for leadership. And to inspire them, he would let them know that the loss of their home, while a huge price to pay, helped to ensure the safety of the world.. This is more my opinion than anything else, but I think it makes the most sense.
(b) This is actually very likely. Barbarians are ridiculously tough men and women, however, everyone has a breaking point. Even the Barbarian hero from Diablo 3 suggests that there are appropriate times to feel sorrow, to cry, and then go take revenge of course.. doesn't he? Isn't it possible that the hero from Diablo 2 has great sorrow.. he succeeded in slaying the demon Baal at great personal risk and sacrifice to only fail to protect the World Stone in the end.. fleeing as its corrupted form is destroyed by Tyrael. It seemed like it was all for naught.. and then to make things worse, this also destroys a huge chunk of his homeland and his people..whom he had been trying to save from Baal and the other prime evils. I think it is highly likely that the Diablo 2:LoD Barbarian had some form of repressed or lost memories...Who would want to remember such things? As important as it was to remember and tell the story, perhaps his psyche snapped and he just couldn't...
(a) This is pretty much the same as (b), same character or not, the Diablo 3 Barbarian hero is old enough to remember the mountain and how it was destroyed. The legends about what occurred during the end of Diablo 2 must have spread across the world and anyone old enough to be from the time surely must know a lot about what occurred, even if not every single detail. But would you want to remember? Would you want to remember returning home from travels to see there is no home to return to and all your family and friends who weren't with you on your travels are now dead. This is of course assuming that the Diablo 2 and Diablo 3 Barbarians are not the same man.
Truthfully, I believe both Barbarian heroes are actually the same man, despite what we've been told:
Not only do they seem to be the same in appearance, age and ability. But much more importantly, the Barbarian hero from Diablo 3 has the ability to summon the Ancients to aid him in battle, proving that he had passed their trials and earned their respect.
If you know anything that would prove me right or wrong, please show me folks.