Why do historians ask a different question of old evidence?

Because evidence can answer different questions.

Suppose historians have a set of Lincoln's letters.

Earlier historians might ask:

What did Lincoln believe about slavery?

A later historian might ask:

How did Lincoln's language about slavery change over time?

Another might ask:

How did Lincoln's statements about slavery relate to wartime political strategy?

The documents did not change.

The research question changed.

That can reveal aspects of the same evidence that earlier scholars did not emphasize.