I asked this a few years ago with no answers -- I'm still curious about it and I figure enough time has passed that someone might have some insight, as this is not something that's been gone into in the books I've read on the subject/time period.
I was reading one of Alison Weir's books about Henry VIII and I realized that I've never quite understood that. She states that Cranmer thought (or said he thought) it was a theological matter rather than a matter of canon law - how did they differ at the time? I would think that theology and canon law would both fall under the aegis of the pope, but based on what I've read that wasn't necessarily the case. Or was it, and was Cranmer just shooting for the moon in terms of possible solutions?
Well, Diarmaid MacCulloch "Thomas Cranmer: A Life" (1998) argues that Henry VIII was obsessed with getting the Pope and Emperor to support his petition. Probably because it was the result that would most validate his claim to moral and scriptural reasons. It's worth noting that Henry was probably not involved with Anne Boleyn when he began this cause, so despite some depictions that Anne was the reason for it all, Henry had conceived a seemingly genuine belief in his own case. Although he slid further away from the Papacy as the divorce proceedings dragged on, Henry held on to the hope that he might get a papal annulment for a long time before giving up on it. So we can see it in that light as part of his quest for approval and a hope for conventional solutions, before he turned to the radical. On Cranmer's part, MacCulloch suggests it was somewhat offhand and he had not necessarily proposed it as a major idea, but maybe something more like a point of principle.
It's worth noting that Cranmer was a noted Cambridge man and humanist, so it was natural to him to offer academic solutions. Although Cranmer does not appear to have been the most political or ambitious, it was also clearly in his interest to present suggestions that would reflect well on him personally. You might say it was a little bit of a "if all you have is a hammer everything looks like a nail" situation. Jonathan Dean's "To Gain at Harvest" (2018) also suggests there was a precedent for this canvassing of universities, although he does not appear to be a historian (he appears to be a methodist priest and university theologian, so scholarly but not historical per se) and he does not say what the precedent was
In terms of it being theology and not canon law, that would firstly be because it was a matter of the interpretation of Leviticus, so it was about the theological basis of the church's decisions, and secondly possibly a foreshadowing of his view that the English Church could settle the matter without the Pope. It's difficult to say exactly how "Protestant" (some historians prefer the term "Evangelical" for early Protestants) Cranmer was in earlier years given the general ambiguity and confusion of the religious issues at that time. But it certainly aligns with Cranmer's later work, including the Collectanea satis copiosa; a kind of primary-source research document presented to Henry that argued the English Church was an independent "empire" the King led. So regardless of whether he meant this earlier idea to lead in that direction, or was more following Henry's lead of a persuasion campaign, he certainly did return to that idea of using academic theological publication.