Why: because the 'minarelli' engine is actually a Yamaha engine and the Yamaha engine is used all over the place in asian markets. The Jog has been a real success in many asian countries. Most "copies" you're talking about probably license builds.
You should also know that Chinese scooter brands have their own (cheap, lousy) engine designs for 50cc 2 stroke scooters, they hardly use minarelli clones for the Chinese mainland market.
Good luck finding a Piaggio scooter in China
Another thing: Since 2004 Chinese scooter manufacturers are switching to 4stroke engines.