How do you adequately explain "opportunity cost” to stakeholders during IT budget conversations, so that budget priorities are successfully financed?

We have a rubric used to evaluate and score IT budget / investments which is used by the entire executive team. The rubric considers risk, strategy alignment, customer experience, efficiency, optimization, breadth of impact, and technology refresh. The nature of 'opportunity cost' is considered by the rubric in that the decision to not do something has been arrived at in consideration of the 'portfolio of opportunities'. The winning initiatives are aligned to deliver the most value. 

If you are meaning, how can I best describe the advantages and benefits which will be missed if the CxO team passes on this opportunity to invest in our IT initiatives? That's certainly a challenge, especially in certain industries which are challenged with lowering overall costs.  In many cases (like cybersecurity), it seems that businesses sell their projects/initiatives based on fear and the potential downside (cyber-event or lost business) if the company doesn't invest.
In the end, I think your role is to continuously beat the drum of improved capabilities and keeping up with competition; but the rest of the CxO team will either be aligned around a sound strategy of investing in IT or not.  If they seem unwilling to be convinced over time, then you must try to find stasis in that environment or find a new company which is better aligned with your approach.

I really hope that did not sound preachy or too black-and-white.  I deeply sympathize with your challenge, but convincing others to do what you see as the best for them can be an exercise in futility because those choices are likely embedded in deeply held values and beliefs.  Those won't easily change.
All the best luck!