We manufacture laboratory equipment. This unit is rated for maximum 40A.

We ask the customer to install a 40A breaker in the circuit. The plug installed is a L6-50. Why is the plug not rated for 40A like the breaker?

This sounds like a homework question to me. Think about it - if a fault occurs would you rather have the circuit breaker tripping or the plug over-heat?

Because a NEMA L6-50 is rated for 50 Amps. It works fine with a 40 Amp breaker. That is what determines the maximum current, not the plug. There is no NEMA L6-40, only 30 and 50 and using a 30 would be a code violation.

PowerStream's Reference Chart of NEMA AC Plugs and Receptacles including drawings and photos