Firstly, because contrary to popular belief, the film going on screen for cinemas is on tape and not in dvd format (far worse quality). It takes time to convert the tape into dvd format, the film producer has to get the rights, get them passed on to retailers, market them, etc, etc. Also, pirating is way to widespread these days so as soon it comes on the market, it gets ripped and people will stop watching it on the big screens. Today's film revenues are still made by cinema screenings and not dvd sales.