The southern Indian Ocean is a remote area with no major shipping lanes. Ships travel slowly so only ships which are already in the area will join the search. The nearest UK or USA ships were probably a thousand miles away.
A chinese ship happened to be in the area and joined the search.
Simple - Anyone nearby will offer help, everything else would take too long to get there.
I am confident that if the USA were asked to provide extra aircraft they would send some, maybe nobody asked.