More may of course be said, but this answer I wrote last year covers your main question reasonably thoroughly. To adapt the key part from that answer:

The establishment of European footholds in China in the form of Hong Kong and the treaty ports after 1842 created legal grey spaces in southern and southeastern China in particular that facilitated far easier emigration from those areas, predating the establishment of European presences on the north coast and in inland China. While it is true that there was in fact already a European holding in China (Portuguese Macao) and a treaty port in all but name (Canton) before the Opium War, these were places where Qing political and legal authority remained paramount. With Hong Kong a place where British rather than Qing law prevailed, and with the treaty ports being a vague, amorphous mess of legal ambiguities, the ability of the Qing to prevent either voluntary emigration or coercive or exploitative ‘recruitment’ was severely curtailed. For instance, the Spanish consul at Amoy just so happened to be a major contractor for organising ‘recruitment’ of Chinese plantation workers destined for Cuba.

This was also helped along by the 'credit-ticket' system, whereby employers covered the cost of transport for migrant workers but would take a cut of their pay until that was paid off, allowing people to emigrate with much less concern for financial status, and so enabling southerners to establish their foothold even faster. This would be strengthened thanks to kinship and native-place associations, as well as secret societies, which grew symbiotically with overseas migration. To quote McKeown:

Even when the institutionalized practice of credit-ticket migration died out in a particular locality, the networks and organizations built on symbols of kinship and native place still persisted and helped channel further flows of chain migration.

The southerners’ early foot in the door created a snowball effect which made it far easier for other southerners to migrate in future thanks to these networks, whereas northerners would be far less able to migrate in the first place and so could not as easily establish the structures that would make it easier for future northerners considering emigration.