The usual factors are: it’s less glamorous so there are fewer collectors; we find more hoards from troubled times; debased coins tend to be more common (they would not be debased if there was no need for more coins).

Yes, the coins were heavily debased during this period, which was one of the main causes of the collapse of the Roman Economy during this period. The Antoninianus coin went from around 40% silver to less than 5% silver in a thirty period from the 240's until the 270's. A great number them were minted in the later time period.