土 on the left grounds 坎 in the earth-and-terrain family alongside 坑 (pit), 堤 (dyke), and 城 (wall). The radical marks it as a feature shaped from soil — here a sunken hollow.
欠 supplies a phonetic that has drifted from qiàn to kǎn but stays in the same rhyme zone. The lacking sense fits faintly: a 坎 is ground that lacks something, a missing scoop of earth.