go extinct

They didn’t ‘go extinct’ after the arrival of Columbus either. Though granted, the Taíno in particular were either intermarried with Europeans or died, and for example their language died out.

But why did they not spread major Old World diseases and cause a massive deadly epidemic?

First, as a caveat, we know a lot less about Viking (or more strictly speaking Norse) settlement of North America - which as far as we know for sure only reached as far as part of Newfoundland - than we do about the early Spanish conquest in the Americas. We do know that the one settlement we know of, L’Anse aux Meadows, itself died out fairly quickly, though it’s difficult to say how long they lived there - or indeed if there was truly a permanently settled Norse population. The sagas are ambiguous but they may have travelled further south, possibly to New Brunswick and even the mainland, and acquired otherwise even then non-local butternuts found on the Newfoundland site.

Second, the Norse in question came from their small settlements in Greenland, which in turn came from Iceland, and in turn from largely Norway before that - all rural, and increasingly remote outposts of few people. These were not the sorts of places to sustain or be rife with major infectious diseases like smallpox, which tended to be kept up and spread in big cities. Renaissance Spain and Genoa had this problem - small rural Scandinavian outposts tended not to, as the small population would have immunity and no large enough ‘new’ population base to spread among.

Third, it’s not just the Norse who were remote: Newfoundland was itself a fairly remote island and we don’t know how much contact the native inhabitants had with elsewhere to be able to spread such diseases, especially with the travel distances involved. From the few records we do have from the Norse side, it seems that contact with the locals was secondary and that, unlike Columbus and the rest, as far as we know they didn’t closely live among them to exploit them but tried to avoid them - not always successfully. The very few records we know include a couple of hostile encounters, with the Norse fleeing to settle far away from them.

That said, we also don’t know much about the population of the indigenous people they interacted with in Newfoundland - people the Norse called Skraelings. They might be identified archaeologically with the Dorset people (in fact so large and sparsely populated is it, that the Norse temporarily inhabited eastern Greenland before the ancestors of the modern Inuit did), and with the remnant Beothuk that the English settlers of Newfoundland met c. 1600, and who died out not long after that. If there was an epidemic brought there, the archaeological evidence is scant enough that we may just not have enough information to tell - not enough written accounts nor rich archaeological sites to be able to reliably estimate population trends over time - but it’s still plausible there was an effect, given the very low population not long after the post-Colombian (or rather post-Cabot) contact. But it’s also clear that the Scandinavians had a far lower chance of bringing it than the later urban Mediterranean folk, and that they’d have a far lower chance of spreading outside Newfoundland than warm trade centres like Hispaniola and other islands where later colonists landed and had a great deal of contact.