It is possible that Permissible is spelt this way rather than any other because of the random nature of some English spellings. In the 1500s there was a push mainly by government-led scriveners to decide upon spelling. The decisions these men made were, in some cases, arbitrary. This may be one reason why sometimes the logic of English escapes us.
Your sentence grammar is not correct. 'Why is permissable not correct spelling?' is correct.
In an English question, generally all the question words have to come first in the question sentence. Ispermissable not correct spelling? Why ispermissable not correct spelling?