Why is there more commercial farming in the western part of Russia?

Kruchkov and Rakovetskaya (1990) found that in the forest zone of Russia, variability of grain production (for the period 1966-1980) reached less than 15 percent in the western part of the country (the Baltic republics and Belarus) and 15 to 20 percent in the central region made up of forest zone and wooded steppes (the Black Earth region) as well as the western part of the Northern Caucasus.