Why is historical accuracy a concern of directors who create historical films?

All of these films would be rendered useless - or actually turn into an insult against all fans or experts -, if they weren't historically accurate, because that exactly is what they promise to be. It is the extra mile that we as an audience expect them to walk: tell us something about a period or moment in time, educate us about how things actually have been, and if you do so properly, we are OK with an extra dash of whipped cream.

Usually these kinds of movies get a personal drama storyline glued to the historical backdrop, and it is more than irrelevant if this storyline is true, altered or entirely fictive as long as the "rules" of the surrounding events are not being hurt.

I guess it is even helpful to connect these historical event movies to the Superhero and Comic Book Movies to understand the relevance of accuracy. On one side, some of the historic events often feature one or more "real" heroes, on the other side, each and every superhero lifeline is tied to a well developed micro-universe with its own rules, premises, laws and ethics. If this gets hurt by a careless storyteller / filmmaker, fans would go berserk and call him a traitor. It is like blasphemy.

To add a final thought: thanks to the internet and its importance for promoting new movies, films are almost completely reviewed and discussed before they even launch. A film that claims to deal with a historic event and then handles history like a list of suggestions would probably get torn apart in advance; which would have a major impact on the "success" of this film, which is exactly what this question is about: box office.