Several factors come to mind. The first is simply that, unlike many prior dynasties, the Ming emerged from a direct resistance movement to externally-imposed rule and had the military strength to preserve this hard-won independence.

Accordingly, there was no sense of kinship between the dynasty's rulers and their (weakened) neighbors on the steppe - who, in past dynasties, were important intermediaries for substantive international relations. This marked a sharp contrast from the way things had been in "cosmopolitan" Sui and Tang, whose founders were deeply enmeshed in non-Han aristocratic networks and routinely married their own princesses to new powers on the steppe. And there was a keen desire among scholar-officials in the Ming to avoid repeating the mistakes of past dynasties, most obviously the Song, which had collapsed due to perceived state weakness in the face of foreign threats.

Another factor weighing against substantive steppe diplomacy, suggested in John Dardess' study of Ming grand secretaries in the reign of the Jiajing Emperor, is that the new powers on the steppe were not all that powerful. After the invasion of the late 1440s when Beijing was attacked by the Oirats and the Zhengtong Emperor was captured, the next major scare didn't occur until Altan Khan's raid toward the capital in 1550. Even here, as Dardess points out, the goal wasn't to conquer the Ming but to harry them. So unlike prior dynasties that, out of necessity, had to interface constantly with surrounding powers to maintain preeminence in a multipolar geopolitical landscape (check out Jonathan Karam Skaff or Wang Zhenping for more on this context), Ming really didn't.

Similarly, as relations with overseas or Southeastern Asian states tended to be structured on the tribute system - in which these states sent goods to China as "gifts" to the emperor in his role as lord of "all under heaven" - this only required proactive diplomacy or engagement from Ming if the system were working incorrectly. Conversely, if tribute was coming in at an appropriate clip, there was no reason for proactive missions to these states - a reason Timothy Brook suggested in The Troubled Empire for the wind-down of Yongle's famous treasure voyages under his heirs.

As Brook also points out, the Ming grew more isolationist over time toward overseas entities (like Europeans, once they emerged on the scene) due in part to piracy on the part of Portuguese and Japanese entities in the 1500s. Over time, wokou raids continued to harry the coast, forcing a military buildup and, as Brook calls it, the treatment of the sea as a "border."