Because a $35 Arduino is within the price range of a $40 Pi; of which both share low level micro controller functions (GPIO on the Pi can function similarly to the Digital / Analog pins on the Arduino). Sure there's significant differences that make a Pi and Arduino board apples and oranges, but at the bottom line, if you've a project that only needs a micro controller that runs a fixed routine that would rarely change go Arduino. If you think you might need that, plus a little more computing power to run an emulator...or to program the low-level directly without necessarily requiring a full computer tower to build and transmit a sketch, it's nearly nothing to pitch a few more $$ and go with the Pi.
Comparing this SoM to the Pi, however... there's no crossover to truly compare. The price points are way too different. There's functions the Pi has that this SoM doesn't (GPIO for example). The SolidRun SoM is more fit to be a low-end desktop replacement than the Pi could ever be. Seriously the SR SoM is more fit to be compared to a low end Laptop ($160 range) as there are many more common points between them, price not being the least of them.