Why not “slave property → factory capital”?

Because those are different kinds of economic relationships.

Under slavery:

enslaved person = legally recognized property of the slaveholder.

After emancipation:

formerly enslaved person = legally free person.

The slaveholder's land may remain.

But the person who was previously treated as property is no longer an asset that can legally be owned.

Therefore the transformation isn't simply:

slave property → factory capital

Instead, several things change simultaneously:

abolition of slave property

• legal free labor
• retention of much existing land by landowners
• new labor arrangements
• continuing inequalities in capital and land

That is why the post-emancipation transformation is more complicated.

“More complicated” here means:

There were several interacting economic and legal changes rather than one simple substitution.