In the construction Why X?, X may be:
A nominal or adjectival describing something in the immediate situation . . . the construction has the sense "What is the cause of X?"
Why the long face? =What is the cause of your long face?
Why so cheerful? =Why are you so sad?
A 'bare' infinitival clause—that is, an infinitival unmarked with to . . . the construction has the sense "Why do you X?" or "Why would you X?", implying that there is a better alternative:
Why wait? = Why would you wait (instead of doing it now)?
Any phrase or clause echoed from the immediately preceding proposition . . . the construction has the sense "Why do you say X?", implying that there is a better alternative or a more appropriate way of phrasing:
A: I'm trying to finish this by tomorrow.
B: Why tomorrow? It's not due until next week.
A: He's obviously exaggerating.
B: Why "exaggerating"? It's a flat-out lie.
So neither of your suggestions is idiomatic. Why passing the opportunity... would make sense only if your interlocutor had used the phrase "passing the opportunity" (which seems very unlikely), and Why to pass... employs a marked infinitival. Moreover, we don't pass an opportunity: we pass it up or pass on it. What you want to say is:
Why pass up the opportunity?