The twenty years referred to is not the length of time that the ark abode in Kirjath-jearim. As a matter of fact, it was there more than forty years. David in his childhood “heard” of it when living in Ephratah (Bethlehem). during his afflictions he vowed to provide a resting place for it. When he became established upon the throne, he found it (Psalm 132) and brought it up to Zion with rejoicing.
The twenty years was the period of divine working in Israel's heart, which led to the complete restoration to the Lord that is now before us. Notice carefully, it was really the Lord they wanted, and no mere symbol, for the ark is not once mentioned in connection with the wonderful doings in Mizpeh. Israel was at that juncture spiritually in advance of the multitudes in modern Christendom who rely upon sacramental symbols, not to mention pictures, images and other follies.
The Lord had delivered His ark from the hand of the Philistines, but He had not yet delivered His people. The time had now come. Note the humiliating contrast with Joshua's day. In his day the people had been able to go forward, conquering and to conquer, no enemies being too powerful for them. Now their highest expectation was that they might be strengthened to cast off the yoke of just one of Canaan's many peoples. Similarly, when we read the book of the Acts with its story of the all-conquering church of God, we lie low as we contrast it with what we behold in our own day.