I don't have it to hand unfortunately and so can't give a comprehensive answer but Frederick Mote's Imperial China 900-1800 spends a fair amount of time on this very question. It is, after all, not just a matter of the Ming: much of the history of Chinese dynasties over the past millennium has revolved around the nomads from 'Inner Asia'.

To give a very quick recap (and I do hope that someone else can expand on this or correct me), it ultimately boils down to the superior military capabilities of the nomads, particularly their far greater mobility. Following their expulsion from China, the Mongol tribes still retained a large and coherent homeland in Mongolia. The collapse of the Yuan had stemmed primarily from a decay in central government and the steppe tribes retained their traditional military advantages that had enabled that conquest in the first place.

So even if they'd wanted to, the Ming would have very much struggled to impose their authority on the Mongol heartland. Generally, the best that China could do was attempt to contain/control the nomads via inefficient static defences. That's not to say that they did that well but I don't believe that extending the border into Mongolia was ever seriously contemplated.

Part of the issue in all this is that we automatically tend to think of settled agricultural societies as more advanced than nomadic ones. Which in many ways is fair enough. But in terms of military capabilities a society of pastoral nomads possessed distinct advantages that the Ming failed to overcome.