Engine choices
Some airlines use "Minimum Cost Cruise" procedures. This sets down a given Mach number to be flown for various weight/altitude combinations and takes many factors into consideration such as fuel burn, flight time, crew costs, maintenance costs etc and, based on data gained over time, the way in which the aircraft is flown is determined to be the most cost effective way. Having various engine choices only adds to overall operating costs against single type in areas of engineering training, spares holdings etc. Most airlines will be acutely aware of how to save a dollar, those who are not don't survive longterm.