cry me a river.
some bands develop and grow in size, some don't.
there's a lot of people who may not have been to download before who are considering it because of the bands booked at this particular time.
the world doesn't revolve around you, get over it. if you don't like it, don't go. simple.
edit: also, just for example, I know there are bands that play more often than not but you get that at other festivals as well. (see Frank Turner at R&L)
but there are other cases who do not -
Shinedown - 2009, 2012, 2016.
Billy Talent (if confirmed) - 2006, 2007, 2009, 2012.
those sort of bookings help sway people