Q:

Why the Black Death took longer to appear in Eastern Europe than in Western Europe

As above, can someone hypothesize why?

A:

The reason was population density and maintenance of proper hygiene.
East Europe was mostly sparsely populated, while larger cities were the worst off, as population densities and close living quarters made disease transmission easier. Cities were also strikingly filthy, infested with lice, fleas, and rats, and subject to diseases related to malnutrition and poor hygiene.
According to journalist John Kelly, "woefully inadequate sanitation made medieval urban Europe so disease-ridden, no city of any size could maintain its population without a constant influx of immigrants from the countryside". The influx of new citizens facilitated the movement of the plague between communities, and contributed to the longevity of the plague within larger communities.
A few rural areas, such as Eastern Poland and Lithuania, had such low populations and were so isolated that the plague made little progress. Parts of Hungary and, in modern Belgium, the Brabant region, Hainaut, and Limbourg, as well as Santiago de Compostela, were unaffected for unknown reasons some historians have assumed that the presence of resistant blood groups in the local population helped them resist the disease.

