That's because every KoM award I've ever heard of involved going up the hill(s). (Note: I know very little about MTB racing, maybe they have one for going down.) The downhill Strava segments strike me, and maybe this is a little mean or judgmental, as something to strive for if you're a Cat 6 [0] racer who can't climb. The first person described in the article was a mid-packer in an actual climbing competition. But when it came to tucking in and staying off the brakes, he could shine at least until tragedy struck.
Mountain biking has plenty of downhill segments. In fact, there is an entire category of the sport conveniently named Downhill.
Downhill racing can definitely be dangerous, but it is my belief that the people who do it are much more experienced than your casual "let's go down this road really fast" amateur on a road bike. Looking down a downhill mountain bike track and set off at full speed takes guts, and no one in their right mind does it without knowing the trail.
One problem is that the default leader board display is just too hard for even semi-decent riders in some areas.
In my area pros come out for early season training and the season opening race. The climb leaderboards are pretty damn competitive - the KoM of one is held by a junior TT world champion, and another by a semi-anonymous rider who rode the Vuelta.
I suspect some changes to the UI (eg show the 10 people ahead of you be default as well as the top-10) could improve this for many people.
[0] http://bikesnobnyc.blogspot.com/2011/05/probing-for-answers-...