Most badges can only be earned once.
Badges are intended to encourage positive behaviour and to introduce the various features of this site. The self-learner badge encourages you to discover the self-answering feature, and to let you know it is perfectly OK to answer your own question. As such, it is only awarded once.
Look at it this way: if the (relatively easy to earn) badge were to be awarded every time you answered your own question, it would instead lead to people creating questions just so they could answer them themselves, regardless of the quality of the question.. That is not exactly behaviour that we want to encourage.
On the other hand, the question-related badges that are awarded multiple times, are awarded for great questions only. They encourage you to ask the best questions you can posit; only if you create high-quality questions repeatedly do you earn those badges again and again. That is behaviour we certainly want to see more of!