Why is relay reliability expressed in MCBF rather than MTBF like other electronic components ?

Relay fallures are primarily due to functional operation (cycling) rather than the amount of time accumulated while operating. A relay manufacturer can not anticipate the number of cycles per hour the relay will be operated when installed in equipment. If the MTBF is required, the relay user can convert MCBF into MTBF by dividing the cycling rate (operations per hour) into the MCBF.