Very poor that no reason given.
However this thread comes to the rescue, as reading the preceding posts, you will note that some TransPacific flights have already operated 12 hours late, been cancelled, or in the case of today's QF11 been substituted with a lower capacity B744 rather than the usual A388. There are only nine of the 12 A388s available at present, but QF's schedules require 10, so something has to give.
I thought with QF now down to seven B744s, it normally had at least one spare, but perhaps not next Thursday.
JohnM, it would be instructive to know when A380 VH-OQA (hit by a catering truck in LAX, and inoperable since 10 June) will be back carrying passengers. (There are two others in scheduled maintenance.) I assume that with your flight cancelled, it won't be before next Friday (21 June 2019.)
Just out of interest, what was your final destination? ORD? MIA? Does this mean a later arrival for you? If so, how many hours?
Perhaps you could ring the QF call centre and ask them why it's cancelled to see what reason(s) are forthcoming.