The use of double pole fusing dates back to the start of public electricity supply. Many places received their supply from the same source as the tramway systems (DC). A 3 wire layout was the norm, +ve outer, -ve outer and midwire (at or about 0v). As a hangover from this time many areas still use a three wire supply. The wiring of buildings allowed the supply to provide 240v (either outer to midwire) for lighting and heating or 480v (outer to outer) for motors. Due to the cost of replacing the networks, there are still in existence, large amounts of these networks. In my early career, I was involved several times in installing equipment for use on 480v. A real pain to do economically and an even larger pain if you forgot the double pole fusing. Many a finger has been warmed by the shock and many a screwdriver blade has been burned off because the neutral fuse had blown, leaving full mains voltage lurking for the unwary. (Been there and done it - several times).
From the "standardisation" agreement in 1928 it was set that the public supply should be a 3phase 4 wire arrangement and that for PUBLIC supply one pole should be effectively bonded to earth (normally the star point of the supply transformer). This provides two functions (1) a reference point for all voltage measurement (2) a means of ensuring effective operation for protective devices in the event of an earth fault.
For high voltage networks the rules are that one point in each network shall be earthed.
As for the original point of "why fuse the positive". I was taught as an apprentice that because the popular analogy for electricity was the flow of water, most people can visualise the flow from a positive bucket (raised to a height) down to a negative bucket (at ground level). Try to convince people that water actually flows upwards from the negative bucket...
Even Tommy Cooper would struggle to get that concept across