Although Duo knows many apostrophised English forms, it graded my "Why's it dirty here?" as 'Correct' whilst stating that "Why's" is 'missing a space'. I've often had similar responses from Duo when I've used common apostrophised English forms (i.e. ones that Duo knows) in the middle of a sentence.
It's simply not a correct word order in English (see br0d4's comment). However, in spoken English, you could omit "it" and ask "Why is here dirty?" - maybe whilst pointing to a small area of the room that the cleaner missed.