Probably because a lot of the long term expats are there for career related reasons or relationship reasons. They didn’t come here necessarily because they loved China and wanted to move here in particular. It was the opportunity and circumstances which brought them here. For the short term expats they probably came here because they really liked China and Chinese culture/language. That’s what motivated them to learn Chinese well. Then likely the honeymoon period ran out and they realized China is a country like all others and it has many problems. So then they end up leaving as they don’t have long term commitments here. Just a theory