Why is "abolition increased labor mobility" plausible rather than a fact?

Because the statement contains a causal interpretation.

Fact:

• slavery existed,
• slavery ended.

Fact:

• people became legally free.

Now:

abolition increased labor mobility

adds a causal claim.

Most historians would regard it as strongly supported and plausible.

But it is still an interpretation about effects.

That is why historians usually distinguish:

• the event itself,
• the interpretation of its consequences.