Troll much? Off topic much? Confrontational much?
You can't answer "why" because
1) you don't know me or my bike.
2) you are making assumptions that may or may not be valid.
Bonus question. Can you think of any fast gravel oriented bikes that can keep up with a road bike on pavement? I can think of 4-5 off the top of my head.
Its not hard for a good gravel bike to be as fast as a road bike in the $1,000-$2000 range (although the gravel bike may be more expensive).
You are right in some regards. Assume I or the OP are on a 25+ lb steel backpacking bike and/or on heavy backpacking appropriate tires and that bike won't be as fast as a road bike at 20-30mph range.
And its right that 15 mph is slow. Most anyone in good shape can do that on a mountain bike.
And yes, gravel bikes don't have to be slow. Again, mine is as fast as my road bike or any road bike in the $1,000-$2000 range. And yes,
@
ljkimmel - if 15mph is a problem its likely the motor, not the bike. A crossrip shouldn't hold you back at those speeds. I don't ride my road bike much any more because my gravel bike is as fast and it is much more versatile.