I usually end up hiking solo because of my career. I'm a surgeon, and I never know when I'll have a patient admitted for the weekend so it's hard to plan ahead, which means that any potential hiking partners will usually have other plans. So my hikes tend to be last-minute affairs and solo. I really don't even try to find partners any more. (I do try to plan one or two 'longer' hikes a year that I take leave for, and that's with an old college buddy of mine.)
But this is why I get annoyed at the occasional idiot who pops up proclaiming that solo hiking is "irresponsible." If I didn't go solo I'd never go. And, frankly, when someone is convinced that a solo hiker is in constant imminent danger that probably says more about their competence and confidence than anything else.
Regarding the AT: The AT is almost more of a 'social' event than a wilderness event. There are groups at most camp sites, for example. I suspect that it's easier for solo women to feel like aid is nearby if they get attacked and scream for help. Of course, I'm probably engaging in a bit if sexism in merely assuming that solo female rarity is due to perceived threats like that. But, for example, my wife feels safer in cities than in the outdoors for exactly that reason- when she's around a lot of people she feels that help is nearby, and thus an attack less likely. :)
OTOH I just ran into a solo woman deep in the Wind River Range last year, though she was a little leery of a group of three males.
