What is the primary reason why European Russia has a higher population than Asian Russia?

European Russia is better suited for agriculture.

Explanation:

The main crops grown in Russia as measured by area cultivated are wheat, barley, sunflower seed, oats, potatoes and rye. The largest share of arable is dedicated to wheat, which with 26.6 million hectares in harvested area in 2009 occupied 21.9% of all arable land in Russia.