Avid Bioservices Inc (CDMO)

[BioBS2012]

Could one explanation be that it is rather rare for 2 biologics[D + B] owned by 2 different companies, neither of which had approval for any indication, to be used in combination therapy?

Possible, but low probability - only because $AZN itself has multiple combo trials running with durvalumab + its CTLA-4 drug, and also other combinations. Same is true for other anti-PD drugs just recently approved (after Opdivo and Keytruda).
IMO the primary reason for the delay was the SUNRISE stop just when the studies were about to commence (last years withdrawn ASCO abstract being an example). Then it was a matter of waiting for all the biomarker analysis to be completed to determine optimal path forward. Given what has been disclosed publicly thus far some good options should materialize in couple of months. Of course the approval of Durvalumab only helps.

Cheers.