Open hand techniques are not teached more regularly because most martial arts are not teached with practical self defense application in mind. Further, most instructors today don't really have a clue on what asian martial arts were about regarding to self-defense. What most people learned in karate was something very different from what it was in the old times of Okinawa, and the intent was actually no longer properly of teaching someone to fight. Of course that doesn't mean karate has become so bad, I think of it as more of an update to a new reality when it was brought from Okinawa to Japan.
As for taekwondo, it's based largely on the karate that was teached in Japan, the same "watered down" version, so the problem remains the same when it comes to using the art in self-defense. I don't mean that someone could not defend oneself with Japanese karate or with taekwondo, I'm sure they can. But the methods used today are not optimal to make the student achieve the skill necessary.
Further, as most instructors didn't really understand traditional methods of self-defense, they stuck to what seemed "strong" to them (or at least what they knew how to use), that is, the closed fist. I'm sure the advent of tournaments have influenced this preference, and western culture -- with boxing, for example -- may also have influenced (you wouldn't be as effective with open hand techniques when wearing boxing gloves). Many techniques were forbidden in tournaments and probably this lead to a dicrease in the emphasis on those techniques, too. Open hand techniques are very efficient when it comes to self-defense, but one has to understand how to use them, that's just it. And yes, a little bit of conditioning is good, but the same can be said about the two-first-knuckles closed fist (seiken in karate). One big mistake is many people don't understand the radical difference between sport sparring (no matter the ruleset) and self-defense, and so they think what is not useful in sparring is not useful in self-defense.
Finally, I use ridge hand, backfist, knife hand and hammer fist as often (or more often) as the more "common" closed fist when sparring in my classes, and I teach my students about those techniques (I also mention spear hand and one/two finger thrusts, but we don't use them in sparring once we aim them at fragile areas of the body -- but we represent the use of them to show they could've been used). I teach KKW taekwondo classes, and all of those techniques are included in KKW syllabus, so nobody can tell me I'm making up things when I teach taekwondo. But that focus on open hand techniques is really particular of my own classes (I'm not the school owner), and also due to the fact that our sparring is focused on learning skills, not on getting points, and thus we don't use any particular rule set -- instead we adapt the sparring scenario to the skill we wish to improve in each session.
One last thing is... the biggest mistake in my opinion on understanding traditional martial arts is the fact that people don't understand forms are mere "formal representations" (that should be obvious, but most don't get it). Instead, they take them literally and try to use the movements exactly as shown in the forms (by the way, today's forms are deluted versions of old forms, and that's relevant, too), don't even thinking of why the movement is shown like that in the forms. So when you have a long front stance that doesn't mean you'll open your legs like that -- instead, you'll understand and use the principle of the stance, that is putting your weight in the front part of your body and putting your weight behind your movement so the movement can be stronger in the forward direction (of course there are other uses for front stance, this is just an example). Every movement in traditional forms teach you really how to fight, not only with simple attacking or blocking techniques, but also with principles, strategies and tactics related to self-defense fighting.