In the early 1800s and previously, ships' guns came in a variety of calibers: 4, 6, 9, 12, 18, 24, 32, and 42-pounders (also 8, 36, and others in various navies). In the pre-Civil War period, the US Navy standardized on 32-pounders, but made them in a variety of weights (and therefore lengths). The classic 32pdr was 57 or 61 cwt (hundredweight) and shorter/lighter versions were 51, 46, 33, or 27. These weights correspond roughly to old-style 24, 18, 12, or 9pdrs; so a ship that might have carried 9pdrs in the old days would have 27cwt 32s.
There were also 32pdr carronades, about the weight of a long 6, powerful but short-ranged, so they had been mostly phased out by the Civil War. We might say the carronade was a victim of its own success, since it stimulated the development of the family of mid-sized weapons. Carronades had been produced in all the standard calibers from 42 (and 68 in the RN) down to 9, not sure about smaller.
With a complete range of weapons in 32pdr, the only need was for something bigger, in the USN the long 42. They could have built a range of 42s, but it would have been needless complication.