Once again, my coming late to the party consists mostly of me saying “what robby said”.
Not so long ago, most cities and towns wanted 24" diameter manhole covers because one person could open it unaided. I have done so myself on a number of occasions. However, as robby said, manholes are considered to be a confined space, so inspections are no longer a one-man operation. (This used to be true only of sanitary sewer manholes, but now applies to storm drainage manholes, and electrical/telephone manholes as well). As a result, many towns are now requiring 30" manhole covers to allow personal breathing apparatus and safety equipment.
A brief and abbreviated description: A manhole is typically located to allow access to the utility it serves. This might be where a pipe makes a bend, where several pipes come together, or after the pipe has continued without interruption for a long distance. They are described as “eccentric”. This means that the top section and manhole covers are not centered on the barrel sections below, but are offset to allow a continuous wall up to the surface. The manhole rungs are then mounted on that wall from top to bottom. And as the old logic question goes, why are the covers always round? They are round so that they can’t accidentally be dropped through the frame and into the structure.
Catch basins, by contrast, are usually “concentric”, with the top section and grate centered over the barrel section, and typically have no rungs. Catch basin grates can be square or rectangular, so if not careful then can be dropped through the diagonal of the frame.
I used to work with an engineer who was extremely circumferentially challenged, and he never had a problem getting into a manhole to inspect it.