That does indeed sound frustrating. Around half of my zoo trips are with family or friends; the rest of the time I go by myself. Visiting alone is an opportunity to do things my way. I visit favourite exhibits multiple times, take lots of photos and have in depth conversations with staff or volunteers. When I visit with people, I like to give them my full attention.
With regards to non committal people - Is the zoo near where they live? Do they have a means to get there? Can they/their family afford to visit? If the answer to those questions is yes, suggest a time/date.
Their response will be:
A) Yes. You suggesting the time/date may have been the push they needed.
B) No. Ask yourself if their excuse is valid? Somebody keen to go will suggest an alternative time/date if they genuinely can’t make it.
C) Maybe. This usually translates to “I’m waiting to see if something better comes up” unless they have a valid reason.
Use your best judgement to determine when somebody is a lost cause. They’re not rejecting you, they’re freeing you up from wasting your valuable time on them.