Here's why:
* Cyclic electron flow is a process that focuses on generating ATP, but not NADPH. It involves the flow of electrons from photosystem I (PSI) back to the cytochrome b6f complex, and then back to PSI. This cycle doesn't involve the reduction of NADP+ to NADPH, which is necessary for the Calvin cycle.
* RuBP regeneration in the Calvin cycle is directly linked to the reduction of NADP+ to NADPH. The NADPH produced in the light reactions provides the reducing power needed to convert 3-PGA (3-phosphoglycerate) into G3P (glyceraldehyde-3-phosphate). This G3P then goes through a series of reactions to regenerate RuBP, which is the primary CO2 acceptor in the Calvin cycle.
In summary: Cyclic electron flow doesn't produce the necessary NADPH for the Calvin cycle to regenerate RuBP. It's a separate pathway primarily focused on ATP production.
