Of great importance was not rewiring or modifying all the engine controls: the new engines had to slide right in and attach to the controls the old engines used. This alone demanded 8 small engines. Read about how they solved the flutter problem on the 747 and you’ll appreciate how important it is not to change the aerodynamics of the wing in any way. Finally, competition for the contract was fierce as there were many different engines that approximately fit the profile: it was very close to competing business jet designs with successful production histories. Rolls Royce, I think, was a surprise winner.