In 30BC, Augustus defeated Antony and Cleopatra at Actium and took over Egypt, integrating almost all of the Greek world into the Roman Empire, and ensuring Roman hegemony over all Greeks. Coincidentally, at precisely this moment in time, Greek literature suddenly became terrible and not worth reading or studying, unless you desperately needed historical information or were a theologian or just wanted to have a laugh/moan at the Greeks.

If you’re capable of detecting the sarcasm within my comment – congratulations! You’ve shown a greater ability to read into something written after the 2nd century AD than pretty much any Classicist/Egyptologist during the 20th century. An essentially circular logic seems to have developed from the 1800s onwards – partially, I suspect, due to the contemporaneous increase in militarisation based on racial supremacy and fetishisation of the Greeks and Romans – in which geopolitical superiority was linked to literary/cultural superiority. So according to this logic, texts written after people were conquered were of less quality because the people had been proven to be of less quality themselves; any study of Greek literature – particularly poetry – tended to be done in order to point out how debased the Greeks had become in comparison to the great literature of Homer and Euripides and Thucydides etc. They must have been conquered by the Romans due to innate flaws in their character, and so their literature, as a product of this character, must be equivalently flawed. There’s also almost certainly a degree of racism involved here: many Greek authors from this period (e.g. Lucian, Quintus, Nonnus) were Greeks born in Asia, rather than the ‘pure’ Greeks that awful 19th/early 20th century scholars (many, perhaps unsurprisingly, German) preferred.

The Alexander Romance, as a Greek work predominantly dating from the Imperial period, suffered the same fate. It’s worth noting, as you say, that it remained popular well into medieval times, and was translated into a huge number of languages (including some that you really wouldn’t expect, like Coptic). Most literature from this time was extensively read and appreciated well into the 1800s, and rejection of it was a fairly recent phenomenon. The other works you listed are more popular essentially because they were composed during ‘good’ periods of history (and are also, of course, very well written works); but if the Metamorphoses had been written after the decline of the Roman Empire, academics probably wouldn’t have bothered with it.

These days, a much more balanced approach to later Greek (and also Roman) literature is taking place, and lots of these works that were deliberately excluded in the 20th century are now being reassessed and studied. Greek epics such as Nonnus’ Dionysiaca and Quintus’ Posthomerica, which offer really interesting insights into Greek experiences of the Roman Empire and their engagement with the past, are now studied for more than just the myths they reference; the Greek novels are gaining popularity again due to their innovations and reflections of the vibrancy of Greek culture (the Alexander Romance fits in here) and its reinterpretation of Greek literature; Lucian of Samosata, who was hugely, hugely popular and influential on modern literature but criticised and rejected in the 1900s, is again being put into the limelight. And so the Alexander Romance, a work with profound importance for many cultures, languages, and literatures across the world, is also gaining popularity again (Richard Stoneman, Krzysztof Nawotka, and Daniel Selden have done lots of work for this), and hopefully will continue to do so, because it can tell us so much about the engagement with Alexander, Hellenism, and the transition between paganism and Christianity by all the different cultures that read it and redacted it.

Secondary References

Nawotka, K. (2017), The Alexander Romance by Ps.-Callisthenes: A Historical Commentary (Leiden; Boston)

Stoneman, R., Nawotka, K. & Wojciechowska, A. (eds.) (2018), The Alexander Romance: History and Literature (Groningen)

Zuwiyya, D. (ed.) (2011), A Companion to Alexander Literature in the Middle Ages (Leiden; Boston)

Selden, D. L. & Vasunia, P. (eds.) (2015-), The Oxford Handbook of the Literatures of the Roman Empire (Oxford)

Stoneman, R., Erickson, K. & Netton, I. R. (eds.) (2012), The Alexander Romance in Persia and the East (Groningen)