Why do winter and summer climates vary more widely in much of eastern Europe?

Due to two factors ...

The first factor is the placement of Europe in relation to latitude lines.

Up north is bound to be more cold than most places down south near the Mediterranean sea. (Which is closer to the equator)

The second factor is the physical geography of the region. Ranging from mountain ranges like the Alps and the Urals to the flat planes of much of mid Europe. The weather interacts with these regions and determines the climates making up much of the continent. (Only takes a rudimentary understanding of basic earth science and geography to understand that. If you have more questions pertaining to the subject, pm me.)