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.