Because two-syllable nouns tend to acquire first-syllable accents in English, while two-syllable verbs acquire second-syllable accents.
See this Wikipedia page about the phenomenon, which includes a list of over 100 words which do this. I remember noticing some words for which this shift seems to be currently in progress, and retard seems to be one of them.
Since the first syllable of the verb is unstressed, the vowel tends to get reduced, and thus changes from /iː/ to /ɪ/ (although some people—me, for one—pronounce the verb with an unreduced vowel). But this vowel change is a secondary consequence of the stress.
Why does this stress shift happen? Once there were enough words that behaved like this, it became a feature of English, which caused more words to undergo this stress shift. I have no idea how it started in the first place, though.