You don't only need fertile soil to grow crops. Just because weather can be favorable to crop growth doesn't mean people actually want to live there. Compare weather patterns.

http://www.marietta.edu/~biol/biomes/biome.jpg

http://en.wikipedia.org/wiki/File:PrecipitationTempBiomes.jpg

http://en.wikipedia.org/wiki/Kiev

http://en.wikipedia.org/wiki/Warsaw

http://en.wikipedia.org/wiki/Moscow

http://en.wikipedia.org/wiki/Berlin

http://en.wikipedia.org/wiki/Paris

Much of the Russian (not eastern European states like Ukraine and Lithuania, not Siberia) territory paints a fairly dreadful weather outlook. Their wettest months of the year are summer months. When the rain lets up a bit, it gets cold. On average, from December till March the temperature wont break 27 degrees in Moscow. From October till April you're not seeing temperatures break 40 degrees. The humidity is always high, averaging around 75% each year, and (at least historically) a lot of the territory was marshy and swampy.

If you looked at your map you'll notice parts of Siberia also have very fertile soil, but no one really lives out there either.

Compare this some well known cities in the US near fertile land-

http://en.wikipedia.org/wiki/Miami

http://en.wikipedia.org/wiki/Los_Angeles

http://en.wikipedia.org/wiki/Eugene,_Oregon

While weather isn't going to be the end-all of why people don't live in places, it helps to explain a lot. I'd wager a lot of people didn't want to live in the Russian territories because it was rather war prone, but as was most of Europe in this period. If it wasn't Mongolians it was Austo-hungarians, or Teutons, or Swedes or the Polish-Lithuanian commonwealth, or Napoleon, or Holy Roman Emperors, or the Ottomans, or Crusaders, or the bloody queen of England stiring up trouble. It's purely annecdotal but my Polish / Germanic roots immigrated to the US back in the days of yore for this exact reason.

Soil fertility isn't everything. Much of Russia, especially west of the Urals, is very cold with a short growing season of less than five months. For example, Novosibirsk, which is the largest city in Asian Russia, only has 4-5 months a year of reliably non-freezing tempratures during which crops can be grown. And that's in the late 20th and early 21st century. During the Little Ice Age, the growing season would likely have been even shorter. Before the introduction of the Potato, Rye is basically the only starch crop that can be reliably grown in this kind of climate.