Essentially because the people of the period didn't, on the whole, live in isolation in the way in which you envisage. The vast majority were not town-dwellers, certainly, but this was not an age in which most farmers owned their own land and lived on it, separately from the rest of the community. Rather, the great landowners, who were for the most part either aristocrats or churchmen, sat at the centre of quite significant communities, often numbering hundreds of people, in which serfs laboured for those landowners, as well as cultivating the land allowed to them in exchange for their service, landowners interacted with all manner of servants, and people also clustered around the providers of core services, whether those were offered by the parish priest, the blacksmith, or the miller – historians such as Carlo Ginzburg have shown that the local mill, for example, was typically a sort of social centre where people would gather to exchange news and gossip.

Nor, with very few exceptions, were these communities isolated from each other. Produce had to be taken into market towns to be sold or bartered for things that were not manufactured in the community; merchants, pedlars and other itinerants moved between communities. So the Black Death – and other contagious diseases – could follow innumerable vectors.

All this is not to say that there were not some societies in which population density was too low, trade too limited, and self-sufficiency too ingrained that the Black Death failed to penetrate them. The Sami culture in Finland, for example, seems not to have been ravaged by the disease. Iceland, too, was sufficiently isolated to be immune from the pandemic's reach. And the low density of population in the central Asian steppes is one good reason for doubting that the Black Death originated in China, as has often been postulated.

But for the most part, even areas of Europe with relatively low population densities suffered terribly from the impact of the pandemic, and the main reason that was so was because the societies of the day were far more closely interconnected than this question supposes.

Source

Ole Benedictow, The Black Death, 1346-1353: the Complete History (2004)