Why is that a sociology-of-knowledge argument?

Because the question has changed.

A historical question asks:

What happened in the past?

A sociology-of-knowledge question asks:

How did social institutions, power, professional communities and intellectual structures influence what came to be accepted as knowledge?

So:

"Lincoln abolished slavery in 1865."

is historical.

But:

"American historical scholarship celebrates Lincoln because American academic institutions favor narratives compatible with liberal nationalism."

is a sociology-of-knowledge claim.

It requires evidence about scholarship and its institutions.