It seems strange to me that at the same time that you see this huge transition in Hinduism from self-reflection, meditation, scholarly practices, etc. to murti puja and the idea of gods as greater powers, the Mauryan empire also heavily promoted Buddhism - which as far as I know rejects the idea of gods being superior and is heavily based on jnana, rather than bhakti, elements. What were the reasons behind this?
Seems to me like if the Soviets were to say "alright we want this country to be communist but private business is also great!"
The question is kind of inherently flawed. The period of the Mauryan empire was marked by a radical shift in religion, but rather than being a move towards the Bhakti that now dominates much of North India, it was the incorporation of Śramanic practices into mainstream Vedic thought and a drastic increase in Śramanic adherence, with Buddhism, Jainism, and Ājīvikism dominating much of North India in terms of politics.
Furthermore, while it is true that Aśoka Maurya was a Buddhist and promoted Buddhism, he did not do this at the expense of other religions, per say. Rather the relative status of the Brahminical elites and the Orthodox rites declined insofar as they were no longer afforded a place above the Śramanas. While this upset some of the more powerful Brahmin families, it was largely a shift in the culture of India that moved away from ritual and orthodoxy to a more contemplative and ethical framework. We must also remember that Aśoka was not the only Mauryan Emperor, his grandfather, Chandragupta Maurya, and his grandson, Samprati, are both honored in Jain tradition for being active missionaries of Jainism and in the former's case, said to have gone forth and eventually achieved Mokṣa. Others in the Mauryan court were Ājīvikas, including possibly Aśoka's father Bindusara and his grandson(?) Daśaratha (although the later was likely a Buddhist.)
With regards to "Hinduism" at the time, we must remember that Hinduism as we know it, that is, the Hinduism with the pantheon of gods and their families related to Viṣṇu and Śiva etc., actually postdates the earliest Śramanas by a century or more. Viṣṇu and Śiva were just moving from minor gods in the Vedic pantheon with no active worship, to the primary objects of devotion at the time of the Mauryas. Furthermore, before this period, the Vedic religion was dominated by adherence to rituals and tradition, with the Upaniṣadic ideals forming a prominent sub-current. Karma as an ethical measure was implemented by the Śramanas, likely the proto-Jains, and this was only then beginning to be incorporated by the Brahminical religion.
As for Buddhism rejecting gods being superior, well that is half-true. Certainly Devas and Brahmas are extremely powerful entities, and could have significant effect on the human world, but they were not all powerful nor omnipotent. While Buddhism rejected the rituals associated with the Vedas on a doctrinal basis, lay Buddhists were not told they could not worship gods or that the rituals could not be done (save for animal sacrifice) but rather that these were not part of the Eightfold-Path.
The downfall of the Mauryas was partially related to religions, at least, according to what we know. About fifty years following the death of Aśoka, the last emperor Brihadratha was killed in a coup and his general, Puṣyamitra Śuṅga, the reasserted Brahminical dominance in the empire, which had shrunk dramatically, losing the West, the Deccan, and Odia. He first performed a horse sacrifice and then took kingship. This was a reassertion of the Orthodox Vedic religion, dominated by ritual and adherence to the social rules outlined in the Vedas. Later Śuṅgas, patronized Buddhism as well however.
In the West, Buddhism grew exponentially, especially in the Indo-Greek areas, while in the Deccan, Jainism and Buddhism were still powerful forces. The slow decline of Buddhism wouldn't really start until the Gupta era in the 4th century CE, when Hinduism begins to look more like what people think of it today. Jainism remained a powerful force even longer in the South.
Further Reading
I also recommend the well done History of India podcast by Kit Patrick and the History of Indian Philosophy podcast by Peter Adamson and Jonardon Ganeri, though I believe the later makes it a bit hard to follow by presenting many later thinkers in Hindu tradition after their Buddhist and Jain antecedents.