Why Did the Jutland Peninsula Develop a "Scandinavian" Culture?

[Filuwaúrdjan]

I don't think I entirely understand the question here. There are significant cultural similarities between North West Germany and Denmark in general and Jutland in particular. There are significant cultural similarities between the pair of them and England. It also happens that the political centre of Denmark eventually became Copenhagen (so open to Swedish influences and, of course, the function capital of both during the Kalmar Union period), that there were always other cultural influences in England, and that the political centre of German, though always mobile, was last in the North West of the country during the transition between the Early and High Middle Ages.

[Filuwaúrdjan]

It's important not to impose certain modern concepts about nation, culture and language onto the Early Modern Period: this was the classic error of 19th century academia.* As Tolkien famously argued almost seventy years ago, you're not dealing with sharp boundaries (whether political or cultural ones) but an endless landscape of marches and hinterlands; everything was so mixed into everything else in all possible senses that you simply cannot say that you have (in this instance) Lower Saxony here, Jutland there, Zealand here and (say) Northumbria there and that they were all entirely discrete regions the cultures of which can be easily distinguished. In an English context we don't know who on earth the Jutes actually were, if they really came from Jutland and quite how they were then integrated into the Saxon polities of Southern England. We also don't quite know how 'Saxon' the furthest West of those (i.e. Wessex) actually was. And so on and so forth.

_{*Well, one of them. One of the worst certainly.}

[Filuwaúrdjan]

It should also be said while the people of the Jutish peninsula of the time spoken West Germanic dialects, the distinction between North and West Germanic were still pretty minor at this point in time, and it’s impossible to find genetic difference Low Germans and Danes at this point in time, showing two population without any admixture barriers (the Swedes and Norwegians on the other had grown slightly distinct thanks to geographic barriers between them and the Danes, so gene flow was primarily from Denmark to the other Scandinavian countries).

Both of these being relevant points for English history in the same general period. Modern genetic surveys aren't quite as reliable as enthusiasts and boosters like to suggest but are still useful as indicators.* Except that in this one very famous case where indicators would be very useful, namely the extent of Norse settlement in the Danelaw, we don't have any meaningful ones as one group of settlers from mostly Jutland turn out to be functionally indistinguishable from another group only a few hundred years later. Meanwhile, while Old English had a West Germanic grammatical structure, Modern English has a North Germanic grammatical structure due to the influence of said Danish settlers and the later literary prominence of writers from Yorkshire and the East Midlands at critical stages in the evolution of the language, though, being a proudly illogical language, not many words of Norse origin outside of the various Yorkshire dialects. Point being that such a change could happen quite gradually almost without being noticed (unlike e.g. the massive influx of Norman French words after 1066) as the languages weren't so far removed at the time.

_{*One fun thing is that we now know there was substantial cross-channel migration entirely unlinked to political drama throughout the period.}