Having had a quick look at the power-to-MTOW ratio of a 789 vs a 744, 77E and 77W the former is lower than all the others (2.5-something with the others being in the higher 2.s), so that might explain the choice.
However, there’s plenty of airlines flying long haul hot & high with 787s; Ethiopian, Aeromexico, Avianca to name but a few. Even BA on the MEX route. I guess there’s an element of forecasted demand but also of limits of the airframe as they are calculated by BA.