All new vehicles have computers....not just the ones that run the engine, but ABS computers, the clock and radio are computers. It takes a minimal amount of energy to keep all these working and up to date. Have you ever disconnected a battery and then noticed all your radio presets are gone, and the engine runs roughly for a short while? To maintain the settings, there is a draw on the battery. If its sits long enough, it will drain to low to crank, a problem exacerbated on a diesel engine which requires a lot of cranking amps.
If it's cold, multiply the problem several times. All batteries lose reserve amps as they cool.
On late model autos, you will either have to run them often, or leave some sort of standby charger on them. Just the way it is.
Ben