TL;DR: Blame the Mongols

In 1440s, the threat of the Mongols in the Northern Yuan dynasty posed a grave threat to the Ming empire, which was further exacerbated with the move of the imperial capital to Beiping (Beijing)which exposed the imperial court itself to Mongol raids. Due to this reality, together with the capture of the Yingzong emperor at the fortress of Tumu by the forces of Esen Choros, Khan of the Northern Yuan, there was an impetus to defend against future Mongol threats at all costs by re-maintaining the entirety Great Wall of China, which diverted lots of funds away from the expensive expeditions of Zheng He.

It was this domestic threat that forced Ming to over-invest in defense and neglect other important areas, like outside trade, which led to an isolationist perspective of the Ming court.

A new source I found on this is "The Perilous Frontier" by Thomas Barfield, which describes the Tumu Crisis in great detail.