I think that the closest answer is: a compromise between visibility and legibility (which of course created some standard with time, or even: best practice).

The main purpose for that is the need to fit as much text as possible, while still keeping the letters quite big (this is why the letters are almost always capital here as well). Fonts used here are mainly 'condensed' subsets of bigger families, or dedicated 'condensed' fonts (you can also find extra-condensed, compressed, narrow or similar naming conventions - you can find more about taxonomy of the type here). One great example is UltraThin Condensed. If a standard font was used here, it would occupy too much space; on a contrary, decreasing the font size of the font would lead to the names being smaller and less visible).

I also believe that, with time, more and more text needed to be put on the posters (so far not-so-important actors didn't want to be excluded from official credits on the posters, and also new areas became important - special effects for example). You can feel it when you look through the posters in the Wikipedia article about film posters.