What exactly did change, that gave people a bigger incentive to convert Steem to Steempower? It's quite simple: higher curation rewards

That's probably part of it. I think another part is encouragement that powering up for the purpose of influence over the visibility and the reward pool (including action against milkers/abusers) doesn't seem as futile as when downvotes were rare and much of the upvoting was dominated by paid votes, self voting, circle voting, etc. The added sense of empowerment is probably an increased soft motivation to power up and curate.