Why do those with pickups feel the need to store boards outside of the vehicle? Or most vehicles for that matter. Do they take up so much space they cannot be stored in the bed? I have a midsize SUV and have them in a bag inside the vehicle taking hardly any usable space.....when I think I may need them. Otherwise...they are in the garage.
Two reasons:
Cleanliness -- Once you use these things, they are filthy. Water is precious so hosing them off isn't an option, so outside the vehicle keeps important gear clean. On Pickups they are often rack mounted on the side because it's an otherwise unused space that generally doesn't block visibility and not much else can go in that space easily.
Accessibility -- they need to be quick and easy to grab for us. If we hit a questionable traction area we are way more likely to grab them and use them thus avoiding getting into a worse situation if they are easy to grab and put to use. If they are buried in the box under other gear, we're more likely to "Let's just try it" and while that sometimes works out, it often doesn't.
We have ours mounted on the side of the bed rack, like the first post with the Taco showed. Our mounting option is home-made using a bit of aluminum plate, some U-Bolts, and some straight bolts. Works like a charm and is very solid.