Lord accepts form (metal, wood, earth etc) to show His mercy to the less intelligent persons

śūdras (the laborer class) and the dvija-bandhus, or the wretched descendants of the higher three classes, are unfit by intelligence to understand transcendental subject matter concerning the spiritual name, fame, attributes, forms, etc., of the Supreme Absolute Truth.  Such persons, although they are unfit to enter into the spiritual affairs of the Lord, can see Him as the arcā-vigraha, who descends to the material world just to distribute favors to the fallen souls, including the above-mentioned women, śūdras and dvija-bandhus. 

Because such fallen souls cannot see anything beyond matter, the Lord condescends to enter into each and every one of the innumerable universes as the Garbhodakaśāyī Viṣṇu, who grows a lotus stem from the lotuslike depression in the center of His transcendental abdomen, and thus Brahmā, the first living being in the universe, is born. Therefore, the Lord is known as the Paṅkajanābhi. 

The Paṅkajanābhi Lord accepts the arcā-vigraha (His transcendental form) in different elements, namely a form within the mind, a form made of wood, a form made of earth, a form made of metal, a form made of jewel, a form made of paint, a form drawn on sand, etc. All such forms of the Lord are always decorated with garlands of lotus flowers, and there should be a soothing atmosphere in the temple of worship to attract the burning attention of the nondevotees always engaged in material wranglings. 

The meditators worship a form within the mind. Therefore, the Lord is merciful even to the women, śūdras and dvija-bandhus, provided they agree to visit the temple of worship in different forms made for them. Such temple visitors are not idolaters, as alleged by some men with a poor fund of knowledge. All the great ācāryas established such temples of worship in all places just to favor the less intelligent, and one should not pose himself as transcending the stage of temple worship while one is actually in the category of the śūdras and the women or less. One should begin to see the Lord from His lotus feet, gradually rising to the thighs, waist, chest and face. One should not try to look at the face of the Lord without being accustomed to seeing the lotus feet of the Lord. SB1.8.22