Part of the reason for this is that many medieval countries don't exist anymore. In period, Spain, France, Italy, and Germany were composed of different nations with seperate languages and governments--just as Britain is composed of England, Scotland, Wales, and Ireland. Most of these smaller countries don't exist as independent units today, so most English-speakers aren't aware of them. But if you begin to investigate them, you'll find many possibilities for personas which are different from the Scots, Irish, Vikings, and Italians who seem to be the largest groups in the SCA.
Burgundy (in eastern France) and Provence (in southern France) were both semi-independent areas which struggled with the French kings based in Paris for many years. Provence had its own language; Burgundy was an English ally against France in the Hundred Years War. Savoy, a nation which contained parts of modern France and Italy, was a nation until the 19th century. Brittany is home to a people related to the Welsh who still maintain their own language and culture.
Portugal was a major sea power, especially in the 15th and early 16th centuries, after which time it was eclipsed by Spain. Despite this there are very few Portuguese personas in the SCA. The Iberian peninsula is also home to the Basques, a group of people who speak a language unrelated to any in Europe. The mountains between modern-day France and Spain, home to many Basques, were the independent Kingdom of Navarre, which might be another source for a persona. The Mediterannean coast of modern Spain was the Kingdom of Aragon, a powerful trading nation who spoke Catalan, a language different from Spanish. The parliament of Aragon met until 1714, and the area (now called Catalonia) is still a center of commerce.
Switzerland, though not a single nation at this time, was a meeting point for French, German, and Italian cultures which few people investigate for personas.
There are sources for personas outside Western Europe which aren't very common. A personal favorite of mine is the Byzantine Empire, which had a great deal of influence over the East. The Byzantines were the Roman Empire, and thought of themselves as such; they were able to survive against great odds until the 15th century. As a Byzantine, your first language would be Greek.
There are plenty of Vikings in the SCA, but few people choose personas from late-period Scandinavia. A late-period Icelandic or Norwegian persona would be an interesting choice.
I hope to add more information about these cultures. For the time being, you can look up a history of these places--the Encyclopedia Britannica should have bibliographies for further resources. If you have acces to a copy of the Encyclopedia of the Middle Ages, you'll find it to be an excellent source. Keep watching this area for updates, and please send in any suggestions.