Jamieson, Fausset and Brown on Deuteronomy 17:17

17. Neither shall he multiply wives to himself, that his heart turn not away--There were the strongest reasons for recording an express prohibition on this point, founded on the practice of neighboring countries in which polygamy prevailed, and whose kings had numerous harems; besides, the monarch of Israel was to be absolutely independent of the people and had nothing but the divine law to restrain his passions. The mischievous effects resulting from the breach of this condition were exemplified in the history of Solomon and other princes, who, by trampling on the restrictive law, corrupted themselves as well as the nation.

neither shall he greatly multiply ... silver and gold--that is, the kings were forbidden to accumulate money for private purposes.