阝 on the left is the mound radical 阜 — distinct from the right-side city 阝 (邑). It marks 阶 as something built into a hillside or earthen rise, the natural site of stone steps cut up a slope.
介 supplies the sound (jiè drifted to jiē). The same phonetic appears in 价界芥. A faint semantic flavor of in-between fits stairs perfectly — they exist as intermediate levels between ground and upper floor.
