If you aren't concerned about voltage drop, power dissipation, or efficiency, then a single diode will do what you want.
Otherwise a single P-MOSFET with a couple resistors makes an efficient and low-drop solution.

 

Hard to know for sure, but it's likely for one or more of the 1 through 3 reasons you listed.

Certainly if the only thing needing protection is reverse polarity, then a MOSFET circuit would be lower loss.

The bridge automatically makes the output polarity the same regardless of which input terminal is positive or negative.

 

In 40+ years I've never seen an MOV and/or GDT in a circuit meant to run from a pre-regulated DC supply - a correctly sized TVS diode is sufficient.
So this circuit might initially have been designed to run from an AC voltage (e.g. 24 V transformer). When regulated DC supplies became cheaper and smaller than transformers, this circuit just survived the transition although it's now somewhat "overspecified".

The bridge automatically makes the output polarity the same regardless of which input terminal is positive or negative.

 

Certainly post #23 makes sense. Since I have not seen the source of the circuit mentioned, it could easily be from the more distant past. AND certainly it looks intended to be a fairly universal input arrangement. AC or DC.