I recall reading or hearing somewhere that the resources/technology/etc of Ming China reached levels even in sea travel great enough for such an expedition, but they simply didn't think about exploring at the time due to border issues.
There's always room for discussion, but perhaps the sections Historians' views of Gavin Menzies' "1421: The Year China Discovered the World" and Travel and contact across the Atlantic before Columbus from our FAQ will answer your inquiry.