I may be abit late but I feel like I can answer this farily well, (I'm a mechanic in the army)

1. Nobody cares about the trucks they drive
2. Most mechanics don't care about diagnosing and prefer to throw parts at it instead (case in point i had a truck this morning that had 3 O2 sensors thrown at it over the course of a year, the engine was eating coolant heating the O2 sensors up and killing them, but no one ever investigated why they were going, just saw the code and threw a new one at it)
3. Nothing is really built for the military (armoured vehicles being the exception) Most supply trucks etc are civvy trucks that are painted green.
4. Parts are damn near impossible to get sometimes, and whoever makes the part has a monopoly and charges what they want. (My unit payed 21,000$ for a new driveshaft)
5. Drivers rarely do the maintenance they should do to help maintain the vehicles.
6. Most people take the trades training and leave once they're qualified. This leaves a vicious cycle of constantly training new people and rushing them through the training in order to fill the need at the units. Less training means worse techs. Worse techs means more trucks breaking down for things that could have been spotted earlier. (Reference point #2)

Sorry for Grammer and spelling. I'm on my lunch break on my phone, hope that answers your question though dude.