That's because compactness doesn't actually tell you anything about unfair voting, demographics, party representation, etc. . . . A district can be shaped like

That's because compactness doesn't actually tell you anything about unfair voting, demographics, party representation, etc. . . . A district can be shaped like a busted IUD and still be perfectly fair and that's precisely why the efficiency gap is so much better at identifying gerrymandered districts. The gap shows the percentage of wasted votes, which is pretty much all that matters.