Many contemporary movie posters typeset the credits in an extremely thin font:
Is this just out of convention or is there a particular reason for deliberately making the credits hard to read?
It's a legal compromise really.
From an article on the New York Times:
So really, in the face of legal obligations this is probably the best they can do. Still just about legible, fulfills the legal requirements to have the key names prominent while also not compromising the aesthetics of the poster artwork too much.
Unfortunately the source article linked to is an image rather than a text so not very accessible. I've transcribed the most relevant element of it above.
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.