Chronic equipment shortages. They would rather delay a while futzing with the current engine, hoping they can fix it, than swap the engine (which does take some time, it's not instant) and then eventually discover in the yard that there was an easy fix for the "broken" engine, but meanwhile they are now down one spare engine. This is compounded by the fact that the old engines are OLD and break down a lot more than they used to, and they've been skimping on maintenance because the old engines will be retiring soon, and the fact that the NEW engines are new and are having teething problems and deliveries are slower than planned. That and they are probably scheduling maintenance by the seats of their pants rather than having good models of adequate preventive maintenance of both the old and new engines and a high likelihood of pulling engines out of service BEFORE they break down.
And add all the breakdowns due to PTC issues, which are still evolving as systems are installed, replaced and upgraded and which they still don't have a good handle on and still don't understand things like why they have to reboot or reinitialize the electronics when certain problems occur. They sometimes discover that all they had to do was power-cycle the magic box or restart things in a particular sequence, and the trouble resolution procedures in the manual don't work but someone remembers someone else once doing it that way and it worked and it becomes folk-wisdom. These are all problems due to having insufficient or insufficiently trained or insufficiently competent support staff for the maintenance departments, i.e. people that train the maintenance and operations staff, and people who develop and write problem resolution procedures for the trainers and the doers.
I've seen these sorts of issue many many times in my own field, and they are mostly due to penny wise and pound foolish management.
