This is something that would be covered much better in the introduction of most translations to his letters. In short, Cicero even in his own day was known as a prose author of indescribably unique ability, and his reputation only grew after his death. His letters were published by his slave, secretary and companion Tiro, and were widely circulated because of his reputation. In the Middle Ages his letters were likewise preserved because they are the best example of Latin prose and so a good resource for instruction. This left them available for the Renaissance humanists.