The hyphen is there because ZK-SNARKs are a subset of SNARKs. Where a general SNARK may leak some or all of the knowledge as part of the proof, a ZK-SNARK can not leak a single bit.
My thesis being, justifying any one particular placement of a hyphen is a necessary but not sufficient part of an adequate explanation. Just as Alice was vexed by Humpty Dumpty.
In other words, my point is that arbitrary and gratuitous hyphens can make it more difficult to understand what ZK-SNARK means. The only system of hyphenation needed is the one that causes people to parse the meaning in the way that the writer intended.
However it is still merely necessary, and not sufficient. When you enumerate the entire set of letter combinations and hyphen positions and assign “lunatic ratings” to each variant, you’ll find many that aren’t so preposterous. Ergo, the chosen form is merely a nominal preference.