Raids tend to have a high level of commitment involved, and a guild in other games (I'm not too sure about TOR) would spend weeks trying to get to the point of being able to consistently complete a raid (or even defeat a single boss), and then farm them for gear. This doesn't lend itself well for the mid-levels, since everyone would be leveling at the same time, so the devs either have to try to get people to do raids for useless gear or you have to give disproportionately powerful gear in the mid-levels.
Also, since raids require more coordination, the leveling process and dungeons below the level cap can be seen as practice for the more demanding raiding scene.
TL;DR: When you're leveling at the cap levels are the advancement you should be going for, when you hit the cap leveling up is no longer possible, so gear becomes the way to advance yourself.
Edit: Since gear can be fully upgradeable, maybe mid-level raids could be used to offer gear only useful for its aesthetics, but I imagine that people would skip them and then come back at the cap when they're trivial.