I would like to see statistics, but it seems almost all Roman coins found are after the 3rd Century. My guess is, Inflation in the Empire was out of control, so there were more coins around, and with chaos and collapse of the Empire, trade and security diminished, so for many folks, the only safe place for wealth was buried in the ground. Are there any studies out there that would confirm or deny this?
The 3rd century was mostly made up of a bunch of independent ‘empires’, each with a series of emperors at war with one another. The economic life of the people of the empire really took off at that time, due to the decentralized gov’ts and their competition with each other, and the need for more money in circulation led to a shift to and big increase in bronze coinage. Those don’t last in the soil like gold and silver does, and tend to be found as a blob of metal and crud that needs lots of care and time to clean up and separate. The practice persisted during the last couple centuries of the Roman Empire, and continued throughout the Byzantine era; the Byzantines got filthy rich due to the location of the capital right on land and sea trade routes, so Byzantine gold coins are (or at least used to be) fairly easy to find and worth more for their meltdown value than for their scarcity.