Low Voltage Switchboards

In the olden days, locating the CTs on the load side of the breaker would allow you to access the CT connections with less risk of electrocution, with the breaker open.

In other cases, CTs on the line side can be used for bus differential protection schemes, where the differential zone includes the circuit breaker, providing extra protection in the case of a fault within the breaker casing.

In a switchboard, it is most likely finding space to mount the CT, a matter of economics in layout and equipment construction. If just simple overcurrent and metering, it most probably is cheaper to put them where they end up. Conventionally, they are mounted on the load side of a breaker, where safety ground clamps can ensure that there is no primary current if the CTs have to have their secondary windings open circuited.