Agreed, a person does have to be healthy themselves before they can be in a healthy partnership. BDSM or vanilla, that's advice everyone would do well to heed. I do think it's likely to see more subs gather under that low self-esteem umbrella, however, do to the broad assumptions about what a submissive is or is not, and the personalities of all types that the label draws to itself.
The only thing submissives really have in common is that we submit. We perform an action that is unique in execution person to person, due to motives also unique person to person.
And frankly, if a lot of the low confidence women to which you're referring ever bolstered up their self-esteem, their desire to "submit" would likely fly right out the window. I hate to say it but as healthy as this lifestyle can be for some, it can be equally detrimental to others. Submission itself can be symptomatic of some very unhealthy thought patterns and processes (and a lot of us ignore that fact because we don't want to admit that what we're having fun with and growing with can actually be kinda' sick).
A few months ago, I had dinner with a co-worker and his friend over at the friend's home. This friend's "girlfriend" set off every subbie radar I have, so at the point the boys wandered off to poke around in the shop I asked her to tell me about the relationship. She acted for a moment like she wasn't sure what I was asking, so I pulled out a picture of J and I in which she could clearly see my collar. She immediately started to spill, talking about how her Owner was a higher class than her, better than her, smarter than her and how she was lucky that he was letting her be there. She talked about her past relationship (abusive of course) and how her Owner had given this last guy roughly six bucks and some clove cigarettes for her.
She and I talk on a somewhat regular basis now, and it comes across as pretty clear to me that she's not happy in her position, per se, just completely unable to envision herself anywhere else. She is baffled as to how I can be a sub in the first place, and beyond that how I don't automatically think my Dom is smarter and better than me.
On the positive side, this girl's Owner is a pretty nice guy, mostly decent to her, but were they ever to part ways (which they will, I'm sure, as I get the impression he's more attached to the live in maid than he is to the person) I can't imagine what would become of her. She's simply not equipped to function on her own. When I've talked to her about things like getting her driver's permit or working outside of the home she's laughed it off by saying, "well if I did those things then I'd be the Owner!"
*sigh* It's a funky world out there...