The difference between Hiura and Frelick is K rate. And it’s a major, major difference. Hiura struck out 26% of the time in AAA and his K rate kept increasing at every level of the minors. Looking back, that was evidence that he had a huge hole in his swing that major league pitchers would be able to exploit, and they did, despite his reputation as a “bonafide hitter”.
Frelick, who already started out as a high-contact hitter, has seen his K rate shrink at each minor league stop, culminating in a 8% K rate last year in AAA, which was one of the best in all the minors.
As for BABIP, yes it’s harder to maintain in the majors due to higher quality pitching and fielding. But we’re talking about a more than .50 point difference here for someone who has always maintained a high BABIP due to his speed. That cries for some positive regression, even if it’s not quite to his minor league levels.