According to Guns, Germs, and Steel, why did the Mongols chose to herd animals rather than farm as their principal source of food?

Diamond does not specifically tell us why the Mongols
would choose to herd rather than farm as their major economic activity.  Therefore, we
must infer this from what is said about other
societies.

Diamond argues that all societies will, if they
are able to, adopt farming because it is more economically advantageous than herding. 
He asserts that if society does not farm it is because it is not able to do so.  This
can be seen, for example, in his explanation of why the Australian Aborigines did not
farm.  This must also be the case with the Mongols. 

To
Diamond, a society can farm if it has suitable native crops and good terrain and
climate.  The Mongols could have borrowed crops from other areas of Eurasia if they did
not have suitable native crops.  Therefore, we can infer that the Mongols' home
territory was not conducive to farming.  The most likely reason for this would have to
do with terrain and or climate conditions. 

Though Diamond
does not explicitly tell us why the Mongols did not farm, we can infer that it must have
been due to a lack of good land or good climate.