It seems simple why historians would object to speculative fictional counterfactionals. Historians are concerned with the real effects of real events. People do

It seems simple why historians would object to speculative fictional counterfactionals. Historians are concerned with the real effects of real events. People do very badly separating fictional evidence from true evidence (bring up AI and see how long it takes your conversational partner to bring up Terminator). As a result, it seems likely that people would discount certain things, or overvalue certain things, based on fictional historical counterfactual evidence. It's also quite possible to consider the impact of an event without fantasizing about what would have happened if it didn't happen; you can see the actual events that it caused and look at their magnitude and effects.