The 380 is for operating to airports with the need to push more pax per slot - preferably at both ends - LHR, NRT and so on.
The 787 is for flights which involve at at least one end a second-tier airport where operating two slots is more efficient than one, like why EI replaced 741s with 767s and 330s way back when ETOPS became a runner.
Fuel burn is important but if you need to get 555 pax out per LH slot rather than a fuel efficient 300 then 787s won't do it for you.