Why is Hindu religion monotheistic?

The gods of modern Hinduism include the chief gods Shiva, Vishnu and the Mother Goddess Shakti as well as a myriad of local community gods. Devotion to these various deities is based primarily on one's region and needs, and even when devotion is given to only one, the existence of others is acknowledged. Despite these polytheistic elements, however, many Hindus explain that the gods are various forms of a single Supreme Being. Similarly, the philosophical Hindu texts advocate a pantheistic view of ultimate reality. These texts, most notably the Upanishads, explain that there exists a single Supreme Reality, called Brahman. Brahman is often personified and presented as the One that must be sought, and can begin to sound like monotheism.

"Hindus believe in monotheistic polytheism, rather than polytheism." (The Hindu Universe)

"Hinduism worships multiple forms of the one God." (OM, an American Hindu organization)

"According to the tenets of Hinduism, God is one as well as many." (HinduWebsite.com)