The thing is, all nouns in French are gendered, and carte (board) happens to be female. So it can only be a mother, and not a father board. And the reason is that it carries smaller boards, like a mom would carry her babies (in most mammal species, as far as I know, the mom is carrying babies even after birth).
A disque (disk) is male. But a floppy disk translate to "disquette" (small disk) which is female : le disque, la disquette. Don't ask me why, but that's who the language works.
This would drive Silicon Valley wokes completely nuts. The table is female, the cloud is male (note that, in Italian, clouds are female). The TV is female, the glass is male, the door is female, the handle is female, the window is female, the roof is male, the wall is male.
[1965 Wireless World July 49/2 (advt.) Contact tail variations..include..tails for direct mounting to a ‘mother’ printed circuit board.] 1965 Electronics 6 Sept. 36/1 Nine daughterboards, each carrying 30 circuit packages, go on each side of a motherboard.
However, that's just an analogy: it doesn't actually tell you anything about what the motherboard does or why. In that sense, I think mainboard is actually a bit more descriptive: it's the principal interconnect between all of the device's parts.