As in why were those of African descent the majority?
In at least one place the slaves were largely Native American, though in part the overall tiny number of slaves contributed to that:
Michigan.
The Michigan Territory had outlawed slavery from day 1 when it was created in 1805. The one loophole in that law was that people who already had slaves were permitted to keep them. The total number of slaves was never more than two or three dozen. The 1810 Census listed 24 slaves, 17 of whom were in Detroit, though that number would go up in following census of 1830 which listed 32 slaves in the territory.
Slavery in Michigan likely began through purchasing from local native groups by French fur traders, and from there it developed into what it was by the 1800s. Upon becoming a state in the 1830s, Michigan abolished what remaining slavery there was, and the 1840 the census reflected that there were no slaves remaining in Michigan.
There's an interesting honours thesis from Eastern Michigan University that goes into some detail, at least more than you're likely to find just browsing Wikipedia and the like. If you're interested, I highly recommend it. I came across it some time ago when I was looking into slavery in my home state and found a lot of what was online to be unsatisfactory. Linked below.
I'm sure there are more in-depth academic papers on the topic, but this at least provides a good overview:
Otherwise I'd echo what /u/samlastname said: It was a problem of numbers.
(Disclaimer: This is obviously outside my area of expertise, but Michigan's legacy of slavery is something I'd looked into out of personal interest some time ago)
They knew the land and knew how to survive off the land so escaping was a relatively easy task. A nation was split into tribes with tribes spread out all over their territory, so even if you captured an enslaved an entire village that would invite the fury of the nation the moment one would inevitably escape and tell the others. They would slaughter anyone using their tribesman as slaves if that happened.
It was far less risky to import people who don't know the land and have no where to go. Although it wasn't uncommon for Natives to take in and adopt escaped and lost African slaves they came across.