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That because carbon is still a new material in cycling, I think they experiment with it in the late 80's and then slowly getting introduced in racing by late 90's (AFAIK), thereforth only exist for a short period of time, not long enough to make a rational comparison with steel/ti.

Does anyone know when carbon started to become the norms in racing?

No. Carbon will definitely go soft/lose strength after a few years. It's the nature of the material. UV light, and moisture are both enemies of carbon, and bikes are exposed to both. We don't need to wait 20 years to know this will happen.