grammar - What's this? What is it? but not What's it? - Why?

Can anyone give a cogent, simply described explanation of why the verb BE in:

• What is it?

... doesn't seem to be able to be contracted with the subject:

• What's it? *

Compare the sentences above with:

• What's this?


• What's that?

These are perfectly fine. In fact, the contractions here should be expected in almost all examples of spoken English.

Bounty edit note: Any answer with any references to authoritative vetted sources will be strongly favoured.

Answer

(1) The word "it" doesn't like to be stressed. (2) Normally, a sentence has its strongest stress on the last thing that can be stressed, which in a simple subject-verb-object sentence will the object, since that is the last thing.

Principles (1) and (2) interact to give the strongest stress on the verb of a sentence, in case the object is "it" -- since the stress can't go on the "it", the last eligible thing for stress is the verb. Compare "I like yoghurt" with "I like it".

(3) Stressed vowels cannot be deleted.

Putting together (1-3), we deduce that the "is" in "What is it?" will be stressed, and consequently cannot be contracted to *"What's it?", because that would require deleting the "i" of "is", which must be stressed because of the following "it".