So the question that remains is: if LW has good material, why does it/do we still target only a very susceptible audience?

This (to me) reads like you're implying intentionality on the part of the writers to target "a very susceptible audience". I submit the alternative hypothesis that most people who make posts here tend to be of a certain personality type (like you, I'm looking for a better term than "personality type" but failing to find anything), and as a result, they write stuff that naturally attracts people with similar personality types. Maybe I'm misreading you, but I think it's a much more charitable interpretation than "LW is intentionally targeting psychologically vulnerable people". As a single data point, for instance, I don't see myself as a particularly insecure or unstable person, and I'd say I'm largely here because much of what EY (and others on LW) wrote makes sense to me, not because it makes me feel good or fuels my ego.

This would probably take a lot of effort (like writing the Sequences for an audience that has trouble grasping fractions), but perhaps it would be worth it?

With respect, I'd say this is most likely an impossible endeavor. Anyone who wants to try is welcome to, of course, but I'm just not seeing someone who can't grok fractions being able to comprehend more than 5% of the Sequences.