Why threshold values of a sensor are readonly

These values have generally been set by the manufacturer and have had service/warranty implications, and therefore were not settable by end users. However, there are certainly cases where they could be.

What are you expecting to do with a settable threshold? We have a Telemetry model currently available as a "work in progress" on the DMTF Redfish standards page that would allow you to generate events when defined thresholds are crossed, for example.

Jeff