I think a couple factors. First, the P&W 772s got delayed coming back into service, meaning United didn't really have any planes to spare. They're now in the summer crush. Another factor I think is that XMN has been in lockdown, which has held up work at that end. Not just the 789s, either. There are currently some 772s in XMN that I would have expected to come out by now. N772UA has been there 80 days, N217UA has been there 72 days, and N792UA for a whopping 115 days. All the widebodies heading into maintenance lately have gone to HKG.

I believe a saw a comment here to the effect that the remaining 789s won't get upgraded til fall now. The only bright spot Polaris-wise is that it looks like the first 764 will finally make it to HKG tomorrow (July 8), after multiple stops and diversions along the way to patch it together. Also the last 763 High-J conversion will re-enter service Friday/Saturday.