Funny you don't understand why: you get the power of the PandaBoard, at the price of two Arduinos.
You probably don't understand because you are comparing the Raspberry Pi with the PandaBoard when it comes to power and to Arduino when it comes to price. Try turning that around. Raspberry Pi is comparable in power with PandaBoard, maybe a bit less powerful, but is half the price!
Raspberry Pi is priced at around two or three Arduinos, but is ten times more powerful. Well, it's not even the same architecture, so I don't understand how you can't understand that someone who would want a cheap 32 bit machine, can't buy an Arduino! And if we talk about RAM, Arduino's can go over a bunch of kilobytes (usually 8), while Raspberry Pi is in the hundreds of megabytes league. Don't you see that your comparing cars to bicycles?