The above answer is based on conjecture and is speculative at best. Slaves in the north were not treated better than in the south. In fact there is some evidence that the exact opposite was true. There were fewer slaves, and therefore different methods of control in the North; yet the underlying cruelty that pervaded the entire "peculiar institution" was present in the north and south. John Hancock and Benjamin Franklin were slaveowners as was the family of William Seward, and Abraham Lincoln's relatives in Pennsylvania. William Penn was likewise a slave owner. There were instances of northern slave owners who forced their slaves to wear iron collars. At the battles of Lexington and Concord, widespread rumors of a slave revolt caused many people to barricade themselves inside and arm themselves with axes and clubs. This hardly suggests "easier lives" for slaves. In fact, elderly slaves in the south could anticipate some comfort and protection from their masters in their last years; northern states/colonies found it necessary to pass legislation to prevent slave owners from turning elderly slaves out into the streets.
There was for many years a concerted effort by northern historians to deny the role of slavery in the North, primarily to saddle the south with the burden of guilt. The quote below gives a typical, though quite false, indication of the myth of northern treatment of slaves. Its bland portrayal of slave/master relations is nothing less than specious:
The slaves in Massachusetts were treated with almost parental kindness. They were incorporated into the family, and each puritan household being a sort of religious structure, the relative duties of master and servant were clearly defined. No doubt the severest and longest task fell to the slave, but in the household of the farmer or artisan, the master and the mistress shared it, and when it was finished, the white and the black, like the feudal chief and his household servant, sat down to the same table, and shared the same viands.
It has been suggested that not only guilt but racism is an element in this denial, as the people of the north wished to minimize the part Blacks played in northern history. Regardless, slaves in the north could not expect better treatment, ran away as frequently as slaves in the south, and ran away as frequently as slaves in the South. They also were forcibly separated from family members by sale. One ad in Pennsylvania offered a woman slave and child for sale "together or separately."
In conclusion, the idea that slaves were treated better in the north than the south is a historical myth. No serious student of history can comfortably embrace it. The links cited below should be helpful.
Although slavery is most connected to the South, there were slaves in the North in at least part of the antebellum period. However, slaves generally had an easier time in the North (of course this was not the same for all slaves) because they were not living on plantations. Slaves on plantations in the South generally had harder lives. The worked in harder conditions and were generally pushed harder than slaves who were not on plantations. Since the Northern economy did not include plantations, the slaves in the North (who were often living in urban areas and in households with only one or two slaves) generally had easier lives than those in the South.