One piece vs two piece has to do with the driveshaft "critical speed", the point at which it begins to swing like a jump rope.
It is affected by length, outside diameter, wall thickness, material, mass and rpm. Highest shaft rpm is affected by vehicle top speed, rear axle ratio and tire diameter. Length is affected by wheelbase and driveline configuration (trans length, t case, etc).
The driveline is also affected by any unusual or unpredicted system resonances, which can crack a Tcase or some other failure mode even at a speed lower than the critical rpm.
From an OE perspective the driveshaft choice (one piece vs two piece) can be manipulated by controlling any one or more of those variables. Vehicle top speed is limited by several different components, including tire speed rating or driveshaft critical speed. Sometimes the driveline is the designated "fuse" (as directed by the program team) and speed is controlled through the use of fuel or ignition cutoff or some other ECM parameter, in order to maintain a one piece system as a cost save or warranty avoidance.
You can also move to a one piece if you are willing to get extreme on the shaft itself: larger diameter, special lightweight materials (aluminum, carbon fiber, "metal matrix"). That's where the piece cost discussion enters in.
You or I (as individuals) might decide to do a one piece shaft figuring we are not going to be driving all that fast.
So, in this particular case, someone (or a group of someones) decided to play it safe and just go with a two piece design, probably because it had always been that way, not realizing they were going to get into some other problems.
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