You have to understand that being diaspora implies something about your identity in the first place. The vast majority of the times, if you liked China and your life in China, you wouldn't leave. Chinese diaspora are typically half-half refugees & economic migrants, in both cases they were not happy with China.
It's only when you consider the 2nd generation, 3rd generation, etc. that there begins to be a sense of "ethnic longing" towards China as a large fraction of the descendants find themselves socially isolated / ostracized in their new, adopted countries. The more conflict with the locals, the worse they're treated, the more they get sentimental about China and develop a desire to identify themselves with their "long lost home land." But by then, it's often too late as they can barely integrate into Chinese society if they went back.
Remember - being a migrant isn't the same as being a colonist. Migrants want to get out of their country, while colonists want to make their country bigger. Migrants want to adopt themselves into a new culture and identity; colonists want to bring their culture and identity to new lands. It's a very different mentality and it's why Chinese colonists in frontier lands are often loyal to China, but Chinese migrants often are not.
