The Vedic religion of South Asia, which ultimately gave rise to modern Hinduism, can be understood as a mash-up of the proto-Indo-European pantheon, which also influenced Nordic/Germanic and Greco-Roman polytheism, with an indigenous Harappan religious substrate that had a somewhat more animistic and a somewhat less anthropomorphic character. It is a process not unlike the process seen in Europe.
In contrast, the Indo-Iranian religion of present day Iran, which appears to have emerged contemporaneously with the Vedic religion from Indo-European contributors very similar to their Indo-European counterparts, was the progenitor of Zoroastrianism, which marks a dramatic departure from proto-Indo-European polytheism, replacing it with what is typically simplified to being described as dualism rather than a polytheistic pantheon.
The open question is, what made Indo-Iranian religion depart so markedly from other Indo-European religious landscapes.
We have a fairly rich trove of written sources, both Jewish and non-Jewish, that sheds light on how Jewish monotheism emerged from Eastern Mediterranean Semitic polytheism to henotheism (in which there are multiple gods but one is pre-eminent and the center of worship) to true monotheism, with some influence from a brief and failed flirtation in Egypt with a monotheistic or henotheistic religion that was heralded by a pharaoh, then abandoned not longer after that by another one.
But in West Asia, we have pretty much only the Avesta, the core and original Zoroastrian scripture, itself to guide us.
One possibility is that the Indo-Iranians had Zoroaster who just happened to be a truly remarkable individual that no other contemporaneous culture had, to make the leap in a way that secured large numbers of followers somehow. Maybe Zoroaster charmed and won over the most powerful kings and princes of West Asia who imposed his religion upon their people. Maybe Zoroastrianism secured a mass grass roots following somehow and was co-opted by West Asia's leaders.
Alternatively, perhaps Zoroastrianism is the legacy of a pre-existing religious tradition and worldview of West Asia that had a powerful influence on the conquering Indo-Iranians, which would certainly not be unprecedented. The Hittites, for example, heavily borrowed from and adopted the Hattic religion of their predecessors, keeping the Hattic language alive as a liturgical language long after it had ceased to be a living liturgical language. The linguistically Semitic Akkadians did something similar with the Sumerian religion. The fact that the language of the Avesta is written in an Indo-Iranian dialect that is a close sister to Sanskrit, somewhat disfavors this hypothesis, but it could be that the fact that Sumerian and Hattic were written languages, while the pre-Indo-European languages of South Asia and West Asia were not, could also explain this fact.
In any of these scenarios, however, there is another point. What about West Asia in the Bronze Age caused Zoroastrianism to fill a spiritual need better filled by dualism than polytheism, that was absent from contemporaneous polytheistic cultures which came under Indo-European rule?
Religions tend to imagine the world of the divine that is parallel to the leadership structure of their own societies in their formative eras.
Animistic religions are associated with hunter-gather societies with weak political structures of fragmentary local bands. Polytheistic religions tended to emerge in chiefdomships where a chief ruled in concern with subordinate but independently powerful other leaders in a sort of council. Egypt's flirtation with monotheism and Jewish monotheism emerged in solitary monarchies with a single clear king or queen. Chinese conceptions of the spiritual world mirrored the vast bureaucracies of its own quick emerging empire.
Was the political situation in West Asia such that the dualism of Zoroastrianism better mirrored its political reality than the chiefdom style of rule of other polytheistic early Bronze Age societies?
Perhaps, for example, the Eastern Iranian region to which the Avesta refers was ruled by a strong unified monarchy, rather than a mere chiefdomship, of the conquering Indo-Iranians, but faced a strong insurgency, perhaps from the pre-Indo-Iranian king who was deposed (perhaps associated with the Jiroft culture) or from an arguably more legitimate heir to the throne who was passed over and was seeking to regain the rule of his dynasty, at the time that Zoroaster formulates Zoroastrianism.
The fact that West Asia was an early place of metallurgy mastery and the Copper Age first and then the Bronze Age is suggestive of the possibility that its political culture may indeed have in some way differed materially from that of its Bronze Age peers, so this isn't an implausible hypothesis.