Conversation so far:
What did Zheng He do in China
Zheng He was a Chinese mariner, explorer, diplomat, fleet admiral, and court eunuch.
why did Imperial officials say Zheng He was not important
It is likely that the expeditions – unauthorized by (and in fact, counter to) the injunctions of the dynastic founder – presented a kind of embarrassment to the dynasty.

Question: why did it decline
Self-contained rewrite: why did Ming naval efforts decline after Zheng He's voyages