We build a large series of boards in various back sizes, so that brings down the cost. It's all about economies of scale when it comes to using motherboards. Passive backplanes can also require more expensive components and upkeep. First of all, since so many cards are packed into a passive backplane system, the slots are closer together than those on a motherboard. Single-board computers also tend to have smaller surface areas than motherboards, so you have to pack a lot more electronics into a smaller package, which in turn makes the smaller board more expensive to produce.
More crowded components pose greater cooling challenges.
You need bigger fans. This means you might be able to extend the life of the board to as long as eight years, says Radisys. So embedded systems really favor the use of smaller motherboards. Motherboards come in several sizes, all based around industry standard form factors. The base NLX motherboard plugs into a vertical riser card with slots for your expansion cards.