As has been said earlier a lot of it is just about what you enjoy, I do both as I like to improve my pb's but personally never get more enjoyment out of running then when out in the hills. It's just so much more interesting in my opinion.
Yes a degree of nuttiness is required for downhill but there is a technique / skill that goes with it as well, some have it, some don't. When new to fell running the climbs are normally a complete shock to the system and it's a case of training / pacing in the races that's required to stop you developing lactic acid on the climbs otherwise by the time you get to the top your legs feel like lead and everyone goes flying past you! Speaking through experience there! 
As for the original question about why fell runners aren't the nuts on the road, well some of the best are but most just like to concentrate on the hills and they are different disciplines. Running on the road is fsater so those who run more on roads are naturally going to be faster than those who spend more time on the hills. That said most serious fell runners (and by serious I don't mean competitive, I just mean those who want to improve) will include speedwork in their training as well to improve their leg speed.
"Feet in the Clouds" - great read, I've done a couple legs of the BG and you've got to admire anyone who can complete the challenge, it would require a massive amount of commitment to do the required training.