Because slaveowners were a smaller percentage of the population than they were in the seceding states.
Slaveowners were only 3% of the population in Delaware and only 12% or 13% in Maryland and Missouri, rather than a quarter or half the population, as they were further South.
Kentucky was a marginal case -- a real swing state where many wanted to remain neutral. It was circumstances -- and the fact that the Confederates invaded the state -- that kept Kentucky in the Union.
Thanks, I’ve been studying the Civil War as a hobby for 20 years and I doubt I will ever understand some things about it.