I was watching the webcast for Falcon 9 Flight 15 (launching DSCOVR) when they scrubbed their first launch attempt due to some issues during the terminal countdown. Before the scrub, the narrator noted that if the terminal count had to be held for any reason, the launch would be postponed to a backup date because the launch window was instantaneous - which is what ended up happening.
I'm curious why the launch window was instantaneous in a mission like this (to Sun-Earth L1). I'm familiar with how launch windows for ISS resupply missions are pretty slim, due to the timing of having to catch up to the station. What factors would cause such a small launch window for a mission of this type?