I primarily specialize in the 12th Century, but also dabble in a fair amount of Arthurian studies. I’m not an expert in Malory, but can offer some insight into this topic.

By the time Malory was writing in the late 15th Century, Arthur was firmly established as the English national mythology, a process that had started in the first half of the twelfth century. I have written quite extensively about Arthur’s transformation into a national hero HERE, HERE, and HERE.

What is ironic about this question is that Arthurian legend began as a Welsh myth. Welsh culture is a Brythonic culture; other Brythonic Celtic cultures include Brittany in Northwestern France, and Cornwall. These cultures share similar languages, and are distinct from the Gaelic Celtic cultures of Ireland and Scotland. For the purposes of this answer, if I say Celtic, I mean Brythonic Celtic.

Medieval Cornwall was still very Celtic throughout the Middle Ages. Much of the nobility after 1066 was comprised of William the Conqueror’s Breton companions. King Stephen would even name a Breton, Alan of Richmond, whose wife Bertha was set to become Duchess of Brittany, as Earl of Cornwall, though Alan proved unable to keep it. Through the interconnection of the Breton and Cornish aristocracies, and relative little direct involvement by the English crown, Cornwall was able to remain more or less Celtic. An interesting point to note, however, is that in 1336, Edward III named his eldest son, Edward the Black Prince, Duke of Cornwall in addition to being Prince of Wales. The Duchy of Cornwall remains a traditional title held by the Prince of Wales, and is currently held by Prince Charles. The title ‘Prince of Wales’ remains a point of contention, as it is representative of the domination of Wales by England.

Tristan/Tristram fits into this because it is widely accepted that Tristan originated from Celtic myth, and was not originally part of the Arthurian Cycle, but was co-opted into Arthurian legend due to the popularity of the story and its harmony with the themes of chivalric literature, which was popular at the end of the 12th C. Tristan’s story is more tied up with real-life locations, like Cornwall and Ireland than many of the other knights, whose origins are more vague, less important to their overall story arc, or are more obviously fictionalized. Tristan’s father’s Kingdom, Lyonesse, is thought to refer to either the Leon region of Brittany, or an Old French name for Lothian (though Brittany seems more likely as there is a strong tradition of the Tristan story in Brittany). In the 1170’s, an early French version of Tristan’s story was written by Thomas d’Angleterre, who places the action in Ireland and Cornwall. Unfortunately, there is a missing link with Tristan in that Chretien de Troyes claimed to have written a version of the story that is now lost.

While it is impossible to say with certainty what Malory’s intentions were, I would argue, based on the above, that Tristan’s Cornish origins are a sticking point for the other Knights of the Round Table because of prejudice towards Cornish culture. While the rest of the Knights have been more Anglicized in Malory’s version, Tristan is cast in the role of the outcast by being Celtic. An argument could also be made that Malory deliberately kept Tristan’s Celtic origins in order to use the literary device of writing from the point of view of the outcast, but he would still be casting Cornwall as the ‘other’ as a reflection of historical prejudice. I wrote quite a bit more on English prejudice towards Brythonic Celtic cultures HERE, though I focused more on Brittany and Wales.

Source

The New Arthurian Encyclopedia, ed. Norris J Lacy. - This book is a bit of a behemoth, and not for casual reading, but it had very helpful entries on Tristan, Lyonesse, Sir Thomas Malory, and other topics of interest. It generally provides overviews of characters, locations, writers, concepts, etc. with a general overview of scholarly perception.