According to River (I think), Lake Silencio was a "still point in time", which made it easier for the Silence to create a "fixed point in time". The term "still point in time" has not been explained in the show. A "fixed point in time" is something that must happen, otherwise time itself will break down (as was seen in "The Wedding of River Song"). The Silence presumably wanted to make what they believed was the Doctor's death a "fixed point in time", so that he'd be unable to avoid it, whatever he tried. As we eventually learned, although the Silence did successfully create a "fixed point in time", it wasn't what they thought it was -- it was the Teselecta impersonating the Doctor and not the Doctor himself that got shot.
In summary: Lake Silencio was chosen because the Silence thought that using that location would make it impossible for the Doctor to avoid their trap but they were wrong.