I'm on Dell Support and they seem to constantly refer to the motherboard as a mainboard or a logicboard. Seems like a strange shift in usage to me.
What happened?
It's just how different companies say things. Apple call it a logicboard, Dell a mainboard/motherboard. The term motherboard is still there and still is commonly used. But the three terms mean the same thing. It's just like calling RAM, memory. It's just a different name but the same meaning.
I called a rep and he said it was because of the way laptops work, they integrate the motherboard with the daughterboards, and it would be a bunch of terminology confusion when someone with decent computer skills came over.
I would say that as some mention its shift from companies terms, and another factor not mentioned yet. The term motherboard has been or mostly been lost confused by many as the motherboard/logicboard whatever you term it now includes a GPU on board on many models and will continue to grow this will confuse the term and when dealing from a support perspective if motherboard means on board video or not?
A few years ago I was told it was because it was politically incorrect (sexiest) so they started referring to motherboards as System-boards, Main-boards, etc
This was around the same time they started calling the cockpit/box-office a 'flight deck'.
Of course, this could have been an old spouse's tale...
Motherboards got more and more stuff added to them until it became something of a misnomer.