I don't know if this has any influence on Korea or Japan's decisions, but here were some "bad experiences" with lopping zeros in Asia; the downfall of the Sukarno regime in Indonesia was closely related to such a decision:
Redenomination was seen as a quick remedy, just as it had been when Germany brought hyperinflation under control after World War I. For Sukarno, however, it proved catastrophic.
"Markets in Jakarta were thrown into bewilderment and became almost panic-stricken," The Nikkei reported on Dec. 15.
Over the next 10 days, the paper reported that prices were rising 10% to 30% in Jakarta, and that army vans were driving around with megaphones calling for people to remain composed and stressing that old bank notes remained valid.
Redenomination merely fueled inflation as stores took to price gouging and stockpiling merchandise. The government threatened merchants accused of such practices with harsh punishments, including the death penalty.
But the price hikes were not simply a case of profiteering. Store owners were forced to raise prices because the government had introduced a "revolution donation" program with the purpose of collecting 10% of the face value of the new rupiah when converted.
The ensuing chaos resulted in the collapse of the Sukarno regime.
Today, however, bureaucrats and senior central bank officials stress that the stability of Indonesia's macroeconomy would insulate the country from a similar reaction.
This was, of course, not a mere redenomination but combined with a wealth tax on cash as well. Some of the communist regimes in Eastern Europe did something similar in the aftermath of WWII, e.g. imposing cash exchange limits for the new currency as a way to confiscate wealth from "exploitative class".
Sinister connotations aside, there's the obvious cost of such a move.
Japan apparently considered it in 2001:
A currency re-alignment would generate a significant amount of work in order to adapt the nation's computer and administrative systems. [...]
But there could also be harmful side-affects, such as opportunistic price rises during the conversion period.
I'm not sure what if any official explanation was provided for dropping the plan.
One 2019 article proposing this idea again does point out that
Japan is exceptional among the major advanced nations in having such a low-valued currency unit. Japan may have the second-biggest economy in the OECD, but its currency unit is near the bottom of the list, at number 17 out of 20. Within the Group of Seven, meanwhile, Japan places last in terms of currency unit value.
But it doesn't mention anything about the failure of the previous attempt.
And one Reuters article made fun of Japan's "quadrillion feat" regarding their national debt.
There are some interesting research papers on the topic as well, e.g. trying to determine the conditions in which the rebasing is likely to occur:
This paper investigates the conditions under which developing and transition nations
engage in currency redenomination. Given that many governments of developing countries
experience high levels of inflation and deterioration in their currencyís value against other
currencies, why do some elect to redenominate, while others do not? And why do some
governments wait many years after a bout of hyperinflation, or after their currency is priced at
1000 or 5000 units to the dollar, to redenominate, while others do so relatively quickly? I
suggest that the explanations rest in a combination of economic and political factors, including
inflation, government's concerns about credibility, and the effect of currencies on national
identity. I employ survival analysis to test these expectations, using a set of data for developing
and transition nations, covering the 1960-2003 period. I find, not surprisingly, that inflation is
an important predictor of redenomination. Redenomination also is related to political variables,
including government's time horizons, the governing partyís ideology, the fractionalization of
the government and legislature, and the degree of social heterogeneity.
Apparently low inflation (such as Korea or Japan experience nowadays) is less inductive to a rebasing, but there are other factors as well.
For instance, in January 2005, Turkey
replaced its currency (the Lira) with the "New Turkish Lira" (YTL), with a conversion rate of one
million old lira to one new lira. And in July, Romania introduced a new "heavy" version of its
currency, the leu, with four fewer zeros. In both cases, governments noted that redenomination
would send a signal to citizens, as well as to the international community, that economic policy
mistakes were in the past. [...] Currency redenomination, then, may come as part of a
broad package of economic and political reforms, as was the case in Afghanistan in October
2002; following years of decline in the currency's value, a new afghani was introduced, with
three zeros removed. This introduction was meant to herald, along with a series of other
measures, the emergence of Afghanistan from years of civil conflict, and its movement toward
So there is apparently a political message that is intended to be conveyed as well by such rebasings. I guess there was a lack of desire in this respect in Korea or Japan, especially in the latter, which had more stable politics.