Why did Europe not see Roman-era numbers of men participating in battles until the 17th century?

Because conflicts that were happening in Europe from the Roman period until the 17th century were not as big. They were mostly smaller skirmishers or conflicts between smaller lords, countries or counties. However, all of this changed with increased access in goods, materials, and production speed that led to a change in the amount of troops that were being deployed to the battlefield in the 17th century.