ImTired
Gravity.
Gravity, while technically the weakest of the four forces on anything but a macro scale, can also affect things in separate dimensions. That means that large celestial objects (stars, planets, black holes, etc.) can affect and do...bad stuff? to ships in hyperspace. I don't even know about nebulas, man. That's just weird.
Also because George Lucas' canon is fucked up. At least Larry Niven had a plausible explanation for why ships in FTL couldn't approach gravity wells
EDIT: Apparently the whole issue with gravity wells has something to do with mass shadows. Whatever.