Mainly because of the control loss, gokudomatic. Self-bondage is a fun idea, but it's also something you're always in control of; you set up an escape beforehand, have a spare key or safety scissors hidden somewhere, or use a timer. There's no point where you're ever TRULY helpless, because if that happens, you're screwed. Hence, stories try to sate the desire for true loss of control, true helplessness: Either something goes wrong and you can't escape, or someone (often but not always a horny friend) shows up when you're tied tight and takes your release away (and often ties you tighter). It doesn't really matter which way they go, overall, since something going wrong will probably lead to the helpless cutie trying to find someone to help her without taking advantage of her (and thus turn into bondage anyways), unless she's especially crafty, it's an especially dark work, or she's just fantasising that something went wrong and she can't escape (which is just a variant of the "actually just a dream" cop-out, more than anything else).
Generally, since it doesn't matter, the story will likely lead to someone else coming to the rescue, by which I mean solving that pesky "fake helplessness" problem by realising the girl's into self-bondage and tying her tighter to help her out. (And probably having some fun with her themself while she's helpless.) And sometimes, the person IS the emergency backup; a common self-bondage tech is to ask a friend to come over a few hours after you expect to free yourself, so they can save you if something goes wrong (with the risk of revealing your secret fetish if they didn't already know), and a few stories do imply this might be why they showed up. (Things like, say, "Well, I called her and she didn't pick up, so I guess I'll drop in and check on her. She's not answering the door, but I hear weird noises inside, and the door's unlocked... oh, my~.") Add in the yuri factor, and that it's an easy way to justify more bondage pics featuring both characters in the future, and the "selfbondage rescue, here to 'free' you by making you more helpless" trope ends up making a LOT of sense.
It also feeds into consensual non-con fetishes, which go surprisingly well with self-bondage. (On the grounds that self-bondage often includes both turning yourself on, and intentionally removing your ability to consent.) It's technically violation, but at the same time is usually exactly what the self-bindee was fantasising about (and thus something she thinks she wants), and the "rescuer" usually being a friend means it'll be consensual ENOUGH (since we know that they would free her if she REALLY wanted it). It makes it easy to segue into the damsel being taken entirely against her will, with no say in the matter, yet simultaneously having exactly enough of a say to be able to veto it if she actually wanted it to end; she can't consent, and she can't say 'no', but her friend is familiar enough with her body language to tell whether she's uncomfortable with what's happening or needs out for real. It ends up being surprisingly easy to play it as the ultimate fulfilment of selfbondage: She's finally well and truly helpless, and the situation is completely out of her control, but she can regain control more-or-less at will if she ever needs to.