Adam Clarke on Exodus 20:25

Thou shalt not build it of hewn stone - Because they were now in a wandering state, and had as yet no fixed residence; and therefore no time should be wasted to rear costly altars, which could not be transported with them, and which they must soon leave. Besides, they must not lavish skill or expense on the construction of an altar; the altar of itself, whether costly or mean, was nothing in the worship; it was only the place on which the victim should be laid, and their mind must be attentively fixed on that God to whom the sacrifice was offered, and on the sacrifice itself, as that appointed by the Lord to make an atonement for their sins.