The reason why historical films are attractive in the first place is because it connects viewers to real people, with real problems. However, what I've found is that Hollywood tweaks history in order to make the movie more engaging. This illustrates that historical accuracy is NOT the reason for how good a film is.
Strong speeches uplift us, because in that moment, we feel like we are being spoken to, and encouraged to "fight on." However, there are plenty of other films in which speeches do not play a huge role in terms of the movie's climax. In that way, viewers are mainly attracted to the speeches in Braveheart and The King's Speech because how much we connect with the protagonist in general.
Therefore the idea of rooting for the underdog encompasses all these facets because it is an overall appeal towards our human desire to connect with people, especially in times of adversity. This video, which is a compilation of several different movies, illustrates my point that even though we might not have experienced what these characters have, we as viewers relate with their struggle because the rest of world seems to be against them. Video credit to myfaith1969