It is very simple:-
First of all, in the past years, 富士康在大陸生產而唔係台灣 as the manufacturing cost in China was lower than Taiwan (big gap). Due to the lower cost, Foxconn can earn higher profit if it put its production in China than that in Taiwan. Foxconn still seconded many Taiwan engineers / trainees / management people to China, the workers and lower/middle level staffs are all China local people.
Secondly, in 80s to 2005 the labour supply in China was unlimited, Foxconn can hire as many as workers it wanted. At the same time in Taiwan, as the Economic in Taiwan grew rapidly, people did not want to do as "worker", the labour supply in Taiwan was limited. (supply<demand)