Why not a GoPro?
Well yeah, a GoPro is great and it's always there, and you can stick it on your helmet etc, but it's only good for the wide shots. The field of view is designed to capture everything and allow you to be quite relaxed about exactly where you are pointing it.
The Tough is better quality, HD, 60fps, and it's not a 'fire and forget' camera. You choose what you want to film. You point it. You hit record. You stop. You put it away.
You've thought about the framing, the angle, the duration. What made a conscious effort to film the right thing first time and not look to find something usable in the edit which is so common with GoPro workflow.
Why not a DSLR?
I'd love my DSLR to be my first choice camera. The quality is great... but it's not in my pocket! For the style of shooting that comes with riding dirt bikes on trips where you've got to get from A to B and not have the luxury of being the camera guy with a van and set up shots etc, then having to get a camera out of a bag, adjust the ISO, aperture, focus etc in the rain and mud is just too much.
Of course, I'm not always restricted to one camera, I often use three, but it's the Tough that's my favourite.