With small, indy films, it's not as big a deal, and they aren't usually announced that far ahead.
With huge movies, like Star Wars, the Marvel films, or Indiana Jones, studios like to plant their stake on their desired future dates so that other studios won't plan releases on the same days.
"Hey, guys, we are taking the week of July 4th in two years, so plan your dates accordingly."
Plus, if you plan it far enough out (like this), you give yourself some wiggle room to move it back if necessary.