Risk Register

Want to understand , is there any specific reason, why the "Risk Register is not shown as an INPUT for Develop Schedule process to add any schedule contingenicies reserves"

In triple constraint (scope, time, cost), the Risk Register is not required for both scope & time. It's required for "cost" contriant only. Why only Cost? Well, you want to consider risks (threats & opportunities) while doing the cost estimates and according to PMBOK not required while defining Scope & Schedule.

Any other insight would be helpful also.