Why is iodine the heaviest bonded element in circumstellar/interstellar space?
I disagree with the assertion that Sodium Iodide has been found in circumstellar or interstellar space.

Sodium Iodide is not on the list Molecules in Space kept updated by University of Köln.

Instead it is likely that someone misunderstood an article such as Interstellar NaI absorption towards the stellar association ARA OB1, where NaI does not mean sodium iodide, but instead means neutral atomic sodium.  

Also, LaO (lanthanum oxide) has been found in stellar atmospheres, and lanthanum is heavier than iodine. See table 5 here: http://www.ucl.ac.uk/phys/amopp/people/jonathan_tennyson/papers/311.pdf

as well as the 1948 article On the Occurrence of Lanthanum Oxide in S-Type Stars.