Frequency choices even then were driven by cost/complexity to the extent that e.g. rather than trying to synchronise things in other ways or decoupling various clock rates, the CPU clock rate of many computer models were chosen based on how other things were clocked.

The Commodore 64's and Amiga's at least were both clocked slightly differently in the US and Europe because the CPU clock affected bus timings, and bus timings affected the custom chips for graphics and sound, and it was most convenient to time it all to match the video refresh rate of NTSC and PAL respectively.

Given that, I'm not at all surprised if the BBC decades earlier found it more complicated to handle 439Hz given whatever clock signals they had available to create it from and/or wanted to be able to match it against.