Okay, your premise is a bit off. There are many frequent assertions made about Cyrus II that are essentially Pahlavi propaganda, that were promulgated by the military strongman Reza Khan (after, with British support, he became Reza Shah in 1926) and his son Mohammad Reza (particularly during the latter portion of his reign, mid-60's and onward). The Pahlavi monarchs were inspired by modernists like Atatürk and Gamel Nasser, and saw to use the prestige of ancient Iran as a way to bolster the prestige of their nation. Bear in mind, while Iran is today an important regional power, urbanized and modern, it was a country in shambles in the early 20'th century, largely illiterate and rural, with about a quarter of its current population.

Whereas Reza Khan/Shah largely focused on projecting a typical image of imperial prestige and pride and antiquity, perhaps roughly comparable to Saddam Hussein's invoking figures like Ashurbanipal in his propaganda, his son was... a bit of a dunce, an insecure figure with a massively inflated ego. For Mohammad Reza, not just the prestige of Iran, but the prestige of the figure of the Shahanshah was important. That is to say, he was concerned with the ancient monarchs of pre-Islamic Iran being reflected in his own person, and in portraying himself as a benevolent figure and constitutional monarch, while brutally repressing dissent. Thus he developed this peculiar obsession with Cyrus and Iranian monarchy as a "2500-year old institution", and in particular, ideas like Cyrus issuing a "charter of human rights" and religious freedoms. This was particularly important for Mohammad Reza in pushing through his white revolution, which sought to "modernize" Iran, undermine traditional classes like the landholding clergy and nobility, improve women's rights and so forth. While he was not as radical as his father (who had outright banned headcoverings in the last 6 years of his reign, with predictable results) his reforms were wide-ranging, not terribly popular, and his stated intent to work in the interest of the poor was undermined by his incredibly lavish spending on things like his "2500-year celebration" of Iranian monarchy at Persepolis, and his lack of concern for Islamic norms (such as spending enormous sums on alcohol for said celebration!).

While his ideas were picked up and are still maintained by some Iranian nationalists, they had perhaps an even more tangible influence in the West; a replica of the Cyrus Cylinder is still, as far as I know, kept at the UN Headquarters in New York. At the time, around 1970, you see the beginnings of the Postcolonial movement and perhaps a degree of wishful thinkings among Western intellectuals when it came to accepting propagandistic claims of modernity by third world nations (skirting the 20-year rule, you could also perhaps find an analogy in the way e.g. Saudi monarchs in more recent years have been eulogized as progressive reformers).

However, while a degree of perhaps excessive positivity toward the Achaemenid Empire existed, Mohammad Reza's claims certainly did not go unchallenged by scholars. For example, a 1983 article by Amelie Kuhrt assesses the cylinder in a serious scholarly manner.

As for classical views of Cyrus, the picture is much more complicated, and it's not as simple as "Achaemenid propaganda", nor does Darius place much emphasis on Cyrus and his family (quite the opposite, in fact). Amelie Kuhrt makes the argument that the image of Cyrus as a ruler of unparallelled wisdom was created largely by Xenophon's Cyropaedia, after it was translated into Latin in the 1440's. As for the Biblical version of Cyrus, there is essentially not much agreement on its provenance or when it was composed; it is similarly unlikely that Cyrus was in any way involved in rebuilding the temple (the book of Ezra seems to suggest it was actually Dareios II who had this done, but the internal chronology of the book is hard to reconcile; it appears to conflate Dareios II and Dareios I, and I've never seen it satisfactorily resolved).

I'd actually suggest reading the aforementioned article by Kuhrt which is freely available on Academia.edu: Cyrus the Great of Persia: Images and Reality as it should give some clarity on the various sources involved.

Let me know if you have further questions!