I haven't seen the "official" release that was given to Guests, but in the info that was given to CMs, under the FAQs if a Guests asks why this was done it is:
"From time to time, we make adjustments to our operations to accommodate for planned transportation maintenance. "
In the past, the Resort line stayed open until the park cleared. So even if there were 5 people in the park, those 3-4 Resort trains are still cycling. Usually it takes around 2 hours for the park to clear after closing. So that saves about an hour each night, and the Resort trains can have a little more time for maintenance.
I'm not saying its the BEST solution, but it will help the trains in my opinion. Nothing would make me more happy than to hear "J/K everyone, we're buying a new set of trains!" But I don't think that will happen soon
12 should be done soon, which will help.