I think it's just that Ponies are really just very innocent creatures, which leads to a large degree of cultural naivete. They don't really pay attention to current events or history beyond what interests them personally and all the other things already discussed.
IMO It's because ponies never have to outgrow a sense of childish, innocent, naivete like humans have to as they grow up. Mostly becuase
1.) Ponies are just such naturally sweet innocent...people? ponies? whatever...so there's not a lot of drive away from innocence from there own society, (I.E. little to no pony on pony crime, violence, con-artists or other things that make people cynical)
2.) Any significantly major threat from without or within is handle by there resident physical goddess. Essentially ponies never have to fully (from the human perspective) 'grow up' and move out of mommy's house.
And I'm not saying it's bad, if anything it makes the whole pony-verse that much sweeter, that it is such a nice place that every-pony there more or less is able to focus on what they love without having to worry about all the little things that plague people turning them jaded and cynical. And yes a side effect of this is the heretofore discussed obliviousness to history, major current events etc if they don't directly pertain to there major interests.
So the Cantorlot elite not knowing about the Elements of Harmony wielders isn't all that un-thinkable, since we didn't see any of them in the celebration scenes, so they could have just chalked it up to, something bad happened but mommy (The Princess) made the bad man go away, and now everything's fine, without really caring about the details. Meanwhile the Ponyville pony's do know about it, but they know the mane 6 don't want any special recognition for it and treat them like normal.
Build a fool proof system, and the world will create a better fool.