There is more to the choice of Stadium than just potential crowd. Firstly, Accor has more corporate facilities than CommBank, the NRL has a lot of corporate partners and would like to ensure as many of them as possible can watch a Final live.
Secondly, there are more facilities for broadcast & media. Again, you want as many journalists, commentators, etc live at the ground as possible whilst also not having to sacrifice corporate hospitality to fit them in.
Rent is cheaper at Accor for matches like this. Firstly for Accor to get the GF, the NRL gets concessions, but also Accor knows more matches helps them retain members (CommBank doesn't have members) and their own corporate partners.
The NRLs deal with the NSW Government also requires Sydney matches in Week 3 of the Finals to be played at VenueNSWs top two venues, which are currently Accor & Allianz.
Even if the above factors weren't at play, the NRL would have felt they could draw more than 29,372 to this match (which is CommBank's largest ever crowd) and I'd be very surprised if they didn't, even without the PSC freebies.