In the course of answering this question I wasn’t able to find a specific answer so for the meantime, allow me to propose a theory. My theory for this phenomena of Burgundy as an entity ending up near the coast of the North Sea started and became apparent because of Philip II (the Bold) of Burgundy’s marriage.

In order to establish the facts, we must review first the background of Philip’s title itself. The duchy of Burgundy separated from its formerly constituent kingdom as per the terms of the Treaty of Verdun, which regulated the partitioning of the Carolingian Empire (per the partible inheritance rule that the Carolingians followed) when Louis the Pious died. The treaty stipulated that the Kingdom of Burgundy would remain Lothair I’s possession as overall Carolingian emperor of the Franks, but crucially, the duchy of Burgundy would be transferred instead to Lothair’s brother, Charles the Bald, who held the title of King of West Francia. [1, p.249] This would precipitate the “migration” of the title of Burgundy away from the kingdom’s nominal position near Provence, Savoy and Avignon.

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Fast forward to 1342, at the time when West Francia became the Kingdom of France, Philip of Valois became Duke of Burgundy. The duchy at this time mostly controlled the areas around the cities of Chalon and Dijon, not far off from the Kingdom’s nominal location. Crucially, Philip’s marriage to Margaret of Dampierre, heir to the County of Flanders, precipitated the migration of focus of the Dukes of Burgundy from Philip onwards away from Chalons and Dijon to the far richer and more profitable county of Flanders, which contained the very rich cities of Ghent, Bruges and Ypres.

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In this assertion, I refer much to the work of Richard Vaughan, who posits that Burgundy, “by medieval standards at any rate, was, from 1384-7 (sic) onwards, a real political and administrative entity. Burgundy was a state. “ [2, p.150] Philip’s shift of focus towards his wife’s domains (and by extension, his domains jure uxoris) is understandable as it allowed him and his descendants the resources in order to fund ambitious projects such as a crusade to Nikopolis (2, p. xxi). But according to Vaughan, the most striking influence of the Burgundian Duke onto the Flemish areas was French administrative practices and centralization efforts. Around 1386 Philip introduced the bailiwick and governor system that he instituted in his own Duchy to the county of Flanders, the county of Artois and their dependencies, mostly in order to streamline taxes and government revenue. [2, pp. 126-127]

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It is my theory therefore that this system, in addition to the fact that the governors sent by Philip to Flanders were mostly French [2, p. 127], created a stark contrast to the relative independent city-state type of governance that the urban population in Flanders enjoyed beforehand, but it was something that worked efficiently, and thus this system that was introduced was kept by the cities in Flanders even after the fall of the Burgundian Valois with the death of Charles the Bold in 1477.

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In conclusion, the reforms the Philip the Good enacted upon Flanders created a Burgundian mark upon the area that was so enduring that the idea of Burgundy as a spatial entity traveled with it.

Sources:

[1] Struggle for Empire: Kingship and Conflict Under Louis the German, 817-876, by Eric Joseph Goldberg

[2] Philip the Bold: The Formation of the Burgundian State, by Richard Vaughan