I will never understand why there are 5 seats and only two JCP candidates. How could progressives reconquer the Senate if we don't even have enough candidates to have a majority ??
Because you can't win five seats.
If you don't try, sure that you can't.
No, it has to do with how the preference math works. If we run too many candidates than we risk splitting votes to the point of not having any viable candidates.
The STV system is done precisely to avoid this threat by chosing the 5 candidates that are most representative of the electorate.
With three major parties, each party is going to at least get one of their candidates in. It's inevitable.