Malbim citing Rav Sadya Gaon: He wanted to nullify the message of the dream, that after gold (Melech Bavel) will come kingdoms represented by silver, copper and iron. He made an image like he saw in the dream, just it was totally gold. He demanded that everyone bow to it, to show that they accept the kingdom of gold forever, that it will never pass. It was a matter of divination and charms; people thought that such matters can change decrees of Shamayim. Therefore, it was only gold, and not metals associated with the other kingdoms.

Rashi citing Eichah Rabah, Pesicah 23: Indeed, they erected it and it fell, until they brought all the gold of Yerushalayim and made a base for it ? "u'Zhavam l'Nidah Yihyeh."

Malbim: Chazal said that this is where Dor ha'Flagah gathered to build a tower. Also they wanted to establish a strong, eternal kingdom, and to overcome decrees of Shamayim and oppose Malchus Shamayim.