It had to do with internal power struggles taking place within the palace, Mandarin bureaucrats vs. eunuchs. Zheng He was a eunuch and the voyages were a symbol of their power and influence. So the non-eunuch bureaucrats wanted to end them. Furthermore there were a number of events that took place towards the end of Zhu Di's reign that caused people to believe that the Emperor had lost the "Mandate of Heaven" (IIRC there were floods, the palace burned down, some other things) and this was interpreted to be a divine condemnation of the supposedly wasteful voyages. So that's why the records were destroyed.