Alternate history becomes fiction when evidence can no longer constrain it

Why does alternate history become so difficult beyond a certain point?

Because the evidence stops constraining the scenario and the historian must invent enough missing links that the exercise becomes closer to fiction writing.

Alternate history becomes difficult when a scenario moves beyond what evidence can reasonably constrain. The analyst must then fill an increasing number of gaps with choices that cannot be demonstrated from the historical record. The work shifts from a historian's analytical method toward a fiction writer's constructive method. Counterfactual analysis remains useful, but this boundary separates tests of history from the creation of a new story.