Why did Martin Van Buren own only one slave?

Though he originally hailed from a slave-holding family, Martin Van Buren came to detest the institution of slavery, calling it "an evil of the first magnitude." Yet Van Buren was also a politician, and as a politician—and a very ambitious one, at that—he knew that there was no way he could argue for the abolition of the peculiar institution and still get elected as president. So he twisted and turned on the issue, devising a series of ever more ingenious compromises which avoided taking the necessary hard decisions.
On a personal level, it is believed that Van Buren owned just one slave, who acted in the capacity of a butler or valet. At that time, it was increasingly difficult to find white people willing to perform the demeaning tasks associated with being a manservant. The industrial economy was growing rapidly, creating more job opportunities for white workers, and so there was a marked shortage of (white) domestic labor. This would've made it harder for upper-class families like the Van Burens to avoid using slaves, even if they themselves found the institution of slavery morally reprehensible.