slavery was important as a cause of the WBTS only to the 5-6% of persons (NORTH & SOUTH) that owned slaves.
free dixie,sw
This figure is a distortion. The 5% number from the South only deals with legal title to slaves while the 5% number from the North (actually, just the four border states: MD, DE, KY and MO plus WV, since slavery was illegal everywhere else in the North) covers the percentage of families that owned one or more slaves.
If we compare the percentage of families in the South that owned slaves versus the percentage of families in the North that owned slaves, we come up with numbers closer to 33% versus 5%, and the numbers for the North are slanted due to the relatively high percentage of slave-ownership in Kentucky.
Put another way, the relevant way to look at this is what percentage of the population in the South versus the North benefitted from ownership of slavery. Furthermore, in the South, you have to factor in all the non-slaveowning Whites who benefitted from slave ownership by others. There was a good percentage of the white Southern population that worked jobs involving slaves (such as plantation overseers).
Slavery was the cornerstone of the Southern economy.