@Vincenzo -- I'm not sure, why that is, but my guess would be it has something to do with fuel economy. If you are in a pickup truck with a big motor, you want to run two-wheel drive as much as you can so you can save gas. Running four wheels takes more power than running four, right?
By the way, one major drawback to those part time, four-wheel drive systems is that you can't use them on pavement. Because all of the wheels are running at the same speed, turning on pavement can be difficult and can lead to a wreck (just check the physics of it to find out why that is -- two wheels travel farther on pavement during a turn and need to go at different speeds).