Just a guess: it's just about economics.
Exactly. Maximum Takeoff and Maximum Continuous are airworthiness limitations, established during certification of engine and airplane, stated as such in the engine and airplane Type Certification Data Sheets (TCDSs), and in the Airplane Flight Manual (AFM). It is an offence against the law to exceed certification limitations.
Maximum Climb and Cruise ratings are contractually agreed between the engine manufacturer and the airframe manufacturer to achieve commercially acceptable targets of time between overhauls and parts replacement costs.
If CLB is equal to MCT the difference is obviously academic.