Well we can assume that it's no reason in particular reason, I mean it's the same thing as having a gasoline can in your trunk. Remember that in space the only way to really get from one place to another is througn warp technology which requires fuel(deuterium)
Deuterium is the primary fuel in both the fusion reactors as well as the warp core. The warp core also requires anti-matter.
Without deuterium the only way the Enterprise could move is if Scotty got out and pushed. Even then he'd need something to push against...
Maybe it's because without Canadian heavy water, the Manhattan Project would never have succeeded.