How could Northerners detest slavery and yet not embrace abolitionism?

First, please note that the vast majority of Northerners
did not detest slavery in the South.  They didn't want slavery in the territories
(because they wanted that land for white settlers), but they didn't really care that
much about black people.  This was especially true in the early years of the
abolitionist movement.  Northern whites did not,as a rule, mind having slavery exist in
the South and did not want a whole bunch of freed slaves coming North to compete with
them for jobs and to, perhaps, live near them.

Even among
those Northerners who truly did detest slavery, there was not complete support for
abolitionism.  One reason for that was that the Constitution allowed slavery.  To
embrace abolitionism, then, was to say that Southern slave owners should have their
legal property taken away from them.  Another reason is that the abolitionists were the
most radical part of Northern society with regard to this issue.  People rarely support
extreme points of view in large numbers.

So, in answer to
this, I would point first to the fact that most Northerners did not hate slavery as much
as this question implies.  Second, I would note that slavery was an embedded part of
American society that had been around forever and was sanctioned by
law.