Pre-clearence isn't open for most of the redeyes - I've flown YVR-ORD before and had to clear at O'Hare since we left around midnight. There are only a handful of flights leaving YVR for the US after 830p (UA to IAH/ORD, DL to JFK, CX to JFK, and the UA/DL flights are summer only), so it makes sense to shut the FIS down in YVR and not staff it for a minimal number of flights when the ORD/IAH/JFK FIS can handle those.

The nice thing about when I took the UA YVR-ORD flight, it meant getting to use the nicer international Maple Leaf Lounge in YVR rather than the transborder one (I was Star Gold at the time)