Why? That's a complicated question. Some words are borrowings or modern coinings, so the positive form never existed in English. Others may have had a positive that fell out of use just because it wasn't a very useful word.
For example, incorrigible was coined around 1340, and corrigible appeared about 150 years later and seems to have lasted through the mid-1800s. The OED hasn't marked it as obsolete or even rare, though it certainly seems to be.
However, I can't really think of an instance when I would really want to use the word corrigible (except to be funny). Even incorrigible doesn't have very widespread use; it's mostly heard when someone's yelling, "You're incorrigible!"
And conversely, there are probably many positive forms that lack a negative.