It's b/c those are the somewhat flex slots. Chancellor does what it's intended for only about 40% of the time while you'll have all 4 of your Chills in your library 60% of the time. Chill just "works" more frequently than Chancellor does, which is one reason for me to include it - less cards that do nothing in most situations. I'd rather have a Lightning Helix than nothing at all (which are roughly the choices at hand).