What is there to conquer? Empires and land grabs typically aren't started for the hell of it. Wars are started because the parties believe that they have more to gain from armed conflict than they stand to lose. Siberia and the Russian Far East don't really have all that much that's worth the effort of administrating. Why bother with thousands of kilometers of empty taiga when there's far more lucrative targets immediately to the south and west?

Even today, much of Siberia has been deemed to be simply not worth the effort of colonising. A 2002 survey found that 26% of Russia's non-arctic areas are untouched wilderness - and that doesn't include the 11% of Russia that's near-empty tundra.

In fact, areas in southern Siberia were the origin of tribute payed to the Mongol courts. Such tribute will in any case have been primitive, consisting of the immediate products of the land.

In my estimation, there are two factors that explain the absence of a decision to extend political control northwards, both of which you topically imply in your question:

• Too little gain - What would political control of Siberia have amounted to? Producing near subsistence levels for most of the year, with no state apparatus or hierarchical structure, even should a conqueror be payed the lip service of formal subordination by all the tribes of Siberia, this would hardly amount to cover even the cost of the campaign needed to subjugate them, not to speak of the cost of collecting minute amounts of tribute from tribes scattered over Siberia's vast expanse and often separated by thousands of miles. For the vast majority of the population of Siberia, nothing would have changed under Mongol 'control' - they might now formally be under a distant ruler, but the affairs of day-to-day living would not be changed. The only thing that the Mongols could perhaps have extracted from Siberia in amortising quantity, at least for shorter campaigns, would have been slaves.

• Too much effort - The northern parts of Eurasia are not very conductive to the mode of warfare that made the Mongols militarily effective (indeed, one could say that it is not conductive to any form of pre-modern, or even modern, warfare). The areas of Siberia that the Mongols forewent conquering coincides almost completely with the presence of boreal forests (taiga). Consider that the expedition of Yermak Timofeyevich in the 16th century was actually slowed by their horses once the Siberian landscape was travelled through. While horses do exist as working animals in Siberia the "Yakut horse" is a breed distinct from the rest of the equine population in its ability to withstand the cold and is descended from animals which did not arrive there until the 13th century. While this shows that horses can be introduced to Siberia's climate, a horse population brought from Mongolia would likely have been subject to severe "evolutionary pressure" before the ability to survive in such extremes of temperature would have become fixated in the population. Further, even the ability to survive the extremes of temperature did not change the fact that horses are, in vast parts of Siberia, not a military asset, even if they can function as draught animals. Not only that, but the paucity of land suited for grazing (and its complete absence in wintertime) would have severely limited the number of horses that could be taken on such a campaign. We see similar constraints working on the Mongol campaigns in Mesopotamia and the Levant. While, militarily, the natives of Siberia would have been no contest and could perhaps be subdued even with the loss of the horse, the boreal climate zones (taiga and tundra) put forth highly specialized cultural demands for survival (as can be seen in the Inuit). The average winter temperature in Yakutsk, an area stretching from just north of Mongolia to the shores of the Arctic Ocean is -30 C° - certainly a factor to take into account for a multi-year military campaign. Logistics would be another problem - human cultures native to this area did not practice agriculture and the population density was (as it is today) very low - while we can assume that the locals could be coerced into paying tribute, it would be hard to feed a large army with it. In that case, it would have been necessary to obtain a large portion of the supplies from the land directly and although the Mongols were equipped and trained for that, the number of troops that could be sustained in this fashion may have been far fewer than would have been required for a military campaign. While the natives would not have presented a substantial mortality factor to an invading army, nature would have run its course.

Summarily, one might say that, during the Siberian summer, a military campaign of short range could conceivably be launched, subjugating a few of the southernmost tribes. This might have happened, prior to the Mongol empire. To excercise effective political control over these tribes would have required stationing representatives of some kind in their own land - who would almost certainly have enjoyed a worse standard of living and would not have had much in the way of decisions to make. Longer campaigns would have floundered in the face of climate and geography.