Just The Facts : Why rollercoasters are never circular

I'm not much of a rollercoasters guy.  I grew up in Edmonton where that one fell off the track years and years ago, and I think that's why I don't trust them.  Knowing that CAN happen is enough to keep me off them.  That being said, if I was at Six Flags or a Disney park or something, I'm sure I'd be all over the rides.  I had never really thought about why rollercoasters don't have complete circles for loops, but by the sound of this, I'm don't want to find out..