There are many reasons for the bias that you've observed. The most important of these is that, as an English-speaker, materials focusing on England are going to be more readily available, especially more obscure works. The same applies to France, mostly because the great narratives of English history have to do with either fighting the French or drawing on French culture for one reason or other (Norman Conquest, Hundred Years' War, etc.). If you were a German medievalist, for example, far more sources on Germany and Italy would be available to you in your native language. This is why any good medieval studies programme requires their students to have a solid foundation in English, French, and German, as well as Latin. The other major element to consider is that the English / American culture has been the dominant global culture for centuries, so English history is going to be taught a lot more outside of England than other histories outside of their territories.

This is not to say that England was not massively important during the Middle Ages. I am often struck by how much power the English kings wielded, especially considering the size and location of the kingdom. Prior to the Norman Conquest, one of the main reasons for England's prominence was its cultural and ecclesiastical output: as an early Christian stronghold in northwestern Europe, Britain was the birthplace of notable missionaries such as St. Patrick and St. Boniface, among many others. This cultural strength was matched by the territorial holdings of the English kings following the Plantagenet acquisition of the Aquitaine in 1137, which, along with their other holdings on the continent, gave them control of half of modern-day France, if only for a time. When one takes into consideration the intermarriage of the English and European royal houses, and the considerable international success of English armies throughout the period, England was certainly a force to be reckoned with.

In terms of primary sources, while I do not have enough information to say that the English were the best record keepers in medieval Europe, they were certainly among the best. This is due both to the litigious tendencies of the English people at the time, and (far more importantly) to the relative stability of the government since the Middle Ages. If it weren't for the French Revolution, which saw the destruction of so much medieval heritage, and other such cataclysms on the continent, medieval England would not be quite so unique in terms of its accessibility. While a number of important buildings / artifacts / documents were lost during the English Civil War, the Commonwealth's general commitment to the maintenance of English culture (Robespierre never would have been buried in St. Denis as Cromwell was in Westminster) saved an awful lot.

This is, of course, just scratching the surface of an extremely complex subject, but please feel free to ask follow-up questions if I didn't answer anything sufficiently, or if you are interested in specific areas that I left out entirely. I hope this has begun to answer your question.