You can only co-op with people who are within 10% of your level range (more if you're in the sunlight covenant), so some people stay low leveled so they can get the most out of co-op. You'll be summoned a lot more at level 40 or 50 in Anor Londo than if you were level 200 for example, because of the average levels of players in that area. If you invade, you can only invade people at your level or higher, so that's another reason to stay low. Also people just stay low level for the challenge or to try out different character builds.
This isn't accurate. It's within 10 levels plus 10% of your own level for co-op (80-120 if you're level 100, for example), and whether you're a sunbro or not doesn't matter.
And darkwraiths can invade players ten levels below their own.