CydKnight
Explorer
My first thought on this is not all Teifling characters are female and I wouldn't want to visualize a mail Tiefling in bikini chain mail but that's just me.My own theory is that it's got to do with the popularity of half-demon chicks in bikini mail.
Visual aid
Of course, the crazy customizability might have something to do with it too. The aesthetics are fun to play with when you've got everything from "elbow spurs" to "antennae" to work with, goth-kid cheesecake or no.
For serious though: What is it about tieflings that gave them staying power over multiple editions? What's their enduring appeal?
A more serious thought is that perhaps it's just fun for people to experiment outside perceived societal norms. The farther the better and what can be further than a spawn with at least one parent, and probably both in most cases, that were pure evil? Even if you don't want to play the character as evil, it can be pretty entertaining coming up with ideas for why this character wants to be the opposite of its parents....and if the evil side comes out from time-to-time that just adds to the excitement. I feel like most of us have probably always wondered, if not actually experimented, with ways to add a touch of evil to their every day lives.