Awesome photo. I really love the depth of field, excellent aperture choice. The trees in the foreground are perfectly blurred to give the perfect sense of grandness of this riding area. Super cool shot man!
Ehhhh... You'd probably be surprised at how many shots this actually took. Riding their bikes? Yes. But organically and not posed? Nope. They weren't just riding through and the photog happened to snap at that perfect time.
A more likely scenario involves radios, 'go!' moments, cues, length of field talk and the riders repeatedly hiking this segment 'one more time'.
Also: to answer your question, the riders are so far away to convey a feeling to the viewer of being out there with them... A fellow rider or hiker, watching them descend, as opposed to an artsy shot of a puddle. Both are great shots, both MTB, but both we're shot under so many different factors (lighting, conditions) to change the emotion of the photo.