The problem isn't the engineering; the Assyrians had notably large gardens which become an important topos in Neo-Assyrian royal inscriptions and hydrological systems such as the Bavian, Jerwan, and Maltai aqueducts built by Sennacherib to provide water to the city of Nineveh(note, however, that the engineering problems here are different because the Assyrian heartland is much less flat than the area of Babylon is). The problem is that we do not have any cuneiform documentation of the Hanging Gardens, leaving us reliant on Greek texts which are rather impressionistic and conflicting, and the lack of archaeological documentation of the gardens(although how recognizable they would be is uncertain). This has lead some academics to simply assume the gardens never existed and at least one recent scholar(Stephanie Dalley) has claimed that the texts that mention the Hanging Gardens in fact refer to the Assyrian royal gardens at Nineveh. I find this argument unconvincing(admittedly as a non-academic), and there's good reason to assume that they aren't utter fictions because they are mentioned in Berossus who had access to the cuneiform texts recording Nebuchadnezzar's reign but it does say a lot about how poorly understood any "hanging gardens" are. It's possible that this was a typical palace garden that was magnified by the Greeks into something more remarkable(as Paul-Alain Bealieau argues); it's also possible that a restrained form of Dalley's argument suggesting that the memory of the Assyrian royal gardens as marvels survived as a topos of royal building which was later applied to Nebuchadnezzar's palace(which may well have incorporated gardens). Anyhow, given that reconstructing lost buildings is often a bad idea with the potential to cause serious damage to an archaeological site if done properly it's no surprise that nobody would want to take the risk of reconstructing a large, poorly documented building on a sensitive archaeological site.