The simplest answer to why they chose to move North is that they were invited.
While the Black community of San Francisco was holding a meeting in their church to discuss their options regarding the State of California's new laws and consider moving to South America an emissary sent by James Douglas, the governor of the Colony of Vancouver Island walked in with an invitation. If they chose to emigrate to the small Colony and swore an Oath of Allegiance to the Queen of England they would become British citizens. They could purchase land, vote in elections and enjoy all of the other rights enjoyed by other British subjects.
The congregation considered this offer and sent a delegation of forty-five people to the colony to investigate this offer and report back. This advance party found the country and climate appealing and the citizens welcoming. They decided to remain and sent word back to San Francisco encouraging the rest to join them. Somewhere between 700 and 800 people accepted the offer and followed them North. (Some history books say 35 men arrived on April 25, 1858 while others record the number as 45. It is my opinion that both numbers are accurate as there are many instances from these days where only men are recorded without mentioning women and children who accompanied them).
These new immigrants became merchants, doctors,carpenters, founded their own fire brigade and even formed a volunteer militia when the American Civil War broke out. Members of their community were elected to the colonial legislature and to positions on the Victoria city council.
One might wonder why the governor and colony felt the need to extend such an invitation to the Black community of San Francisco. The colonists were decidedly anti-slavery and had a previous record of rescuing slaves from bondage. A few years earlier they heard of a young slave boy who was being mistreated by his master. They sent a patty to Nisqually which kidnapped the Boy Thomas and brought him back to Victoria with them. British law at the time declared all slaves were released from bondage upon setting foot upon British soil so the young lad became free. On another occasion the merchants of Victoria learned that there was a slave on board an American ship docked in their harbour. A group of them went to the dock and encouraged the slave to join them while working on the wharf. As soon as he set foot on the wharf they seized him and gleefully carried him into town while declaring him a Free Man. Incensed, the slaves master filed a complaint with the local police and a court action through his government. He was not successful in his challenge to his right of ownership.
One should also note that James Douglas, a man with the dual capacity of being the governor of the Colony and head of the Hudson's Bay Company's holdings in the Pacific Northwest was a man of mixed lineage. His father was a British merchant and his mother was a mulatto from the Caribbean. His wife was the half-breed daughter of Northwest Company Factor William Connolly and his Crew wife.
While these seem to be very noble reasons to invite the Black community of San Francisco to Vancouver Island there are other factors as well.
Significant quantities of gold had been discovered on the Thompson River on the mainland next to Vancouver Island the year before. The natives along the river were picking up shiny rocks and exchanging them for goods at the nearby Hudson's Bay Company's trading posts. They were given spoons and encouraged to dig up more and news of this discovery was sent to James Douglas, the man in charge of the region. Douglas encouraged the traders to keep this discovery secret while he communicated with the company's board of governors back in London. During the same summer of 1857 a group of American prospectors also reached the Thompson River and discovered gold as well. They entered British soil by land via the Columbia River and Okanogan region and returned to Astoria Washington the same way that fall.
Douglas heard about this group of prospectors and realized that the volume of gold in the Thompson River area would be significant. In his writings to the British Government's Colonial Office in Westminster and the HBC Board of Governors he said that he expected a large number of miners soon heading north and that they would encourage their American government to annex the mainland of what is now British Columbia.
Douglas was second in command under Dr John McLoughlin when the Hudson's Bay Company lost possession of its Columbia River holdings. He soon advanced his position and became in charge of the region after Dr John retired from the fur trade. Douglas saw first hand the resentment towards his company and Crown and spent much of his time embroiled in diplomatic tangles with the Oregon people.and their government. He understood the diplomatic perils facing the British possessions as well as his own company's future owing to its Small population.
The Colony of Vancouver Island had less than a thousand inhabitants (of British/European descent, of which 500 were settlers) when the New Year rolled in in 1858. There were fewer than 500 men working for the Hudson's Bay Company on the mainland and many of these were seasonal workers who would head east in the fall. The arrival of the settlers coming from San Francisco almost doubled the population of the colony overnight.
It is doubtful to say that the arrival of the new immigrants from San Francisco strengthened the colony's political strength though. The ship which carried the messanger with the invitation to the Black community of San Francisco also had a cargo of 800 ounces of gold destined to the San Francisco mint. This shipment was supposed to be kept secret, but news like this is impossible to keep quiet. When they landed in Victoria on April 25 of 1858, the advance party sent to scout out the region were not the only passengers on board the ship. As they disembarked from the Commodore they were among 400 gold miners on their way up the Fraser River to the Thompson River diggings. By August of 1858 the population of Victoria had swelled to 20,000 and it reached 30,000 the following year