While the Vikings originally came from Norway, Sweden and Denmark, they had a rather wide reaching effect on the world at the time they lived in it. Take a look at the names of the cities founded by the Scandanavian civ. It will give you some idea of the impact they had on the whole of northern europe. Their raids were feared throughout northern Europe. The "vikings" settled permanent cites as far away as Iceland, and were the first peoples to find, and live in the New World...a full 500 years before anyone else made it there. If it weren't for the indiginous peoples of what is now Newfoundland, Canada might also be decendant from a scandanavian nation. There were Viking states in Ireland, England, Scotland, the Netherlands, France, Turkey, Faroe Islands, Iceland, Greenland, Russia and the Ukrane.
Now, while many other peoples have had wide reaching effects on the world, I think the point was alread made that not everyone can be included. Civs were chosen (I think anyway) not only on their "historical" merits, but also on how well known/ popular they are in a global sense. While this is purely an anecdotal example, in my history classes (in the dim, dark past

), I was taught something about most of the civs in the game, but no one that I had to read about was left out...popularity does have its advantages I suppose