Why must 'galore' be used postpositively?

There are several post-positive adjectives. In some languages post-positive adjectives are commonplace, but as you note, in English they are rare, limited to archaic or institutional expressions. Some adjectives have a different shade of meaning if used post-positively.

There are many set phrases such as "best room available", "words unspoken", "the light fantastic" which use post-positive adjectives.

The word galore comes from the Irish go leór, corresponding to Gaelic gu leóir meaning "sufficiently, enough". I agree with the explanation in Jon Hanna's excellent answer to this related but closed question: What is the etymology of “galore”?

Such post-positive adjectives are unusual, and all the more unusual as the general way to use the word, rather than as a few cases of historical significance or poetic origin (e.g. we use "Choir invisible", but otherwise invisible is not post-positive).

This reflects its being a relatively recent borrowing, much as par excellence (another adjective normally used post-positively) is a relatively recent borrowing from French. Forcing English into the patterns of Irish, suggests that those to first make use of it in Irish were more familiar with that language, than English; that is to say, it was borrowed by native speakers of Irish, rather than by Hibernophile Englishmen.