This is somewhat covered in the FAQ: https://www.reddit.com/r/AskHistorians/wiki/historians_views#wiki_historians.27_views_of_gavin_menzies.27_.221421.3A_the_year_china_discovered_the_world.22
Also look to this previous thread : https://www.reddit.com/r/AskHistorians/comments/1rtw8b/why_didnt_the_chinesejapanese_or_other_countries/