Climate is the biggest reason. Western Russia is simply much warmer than the eastern part. Predominant winds in mid-latitudes are west-to-east, which mean that western Russia experiences a significant influence of the Atlantic Ocean and the Gulf Stream. Not to the extent that Western Europe does, but enough to make it quite livable. (You can observe a similar phenomenon in North America. For example, you will not find large cities eastern Canada at the latitude of Calgary or Edmonton.)

Eastern Russia is cold. Like, really cold. Vladivostok, one of the southernmost Russian cities on the Pacific coast and on a similar latitude as Nice, France, has significantly colder winters than Moscow, which is at the latitude of Scotland. Very harsh winters and short summers make affect everyday life. Permafrost makes building infrastructure more difficult. Soils are poor because the biome is dominated by the coniferous Taiga forests. Physical isolation adds set of problems. Most of the rivers in eastern Russia outflow into the Arctic Ocean which isn't navigable for almost the entire year. So geography and climate are the biggest challenges.