There are valid reasons why it isn't for the weak.
For one, Laveyan Satanism which a lot of satanic cults and philosophies sprouted from... is all based on Survival of The Fittest.
Now for the other reasons.
One of them is the concept was already stated, everyone has their own weaknesses and strengths. It is a matter of overcoming those weaknesses that a lot of luciferian/satanic paths are about. This is because the LHP is all about Self-Mastery.
Another reason is that a lot of people have been too far conditioned to really follow the LHP. Conditioning from society has led us to want to follow ''happy go smile at yourself in the mirror'' religions. They have completely lost touch with the dark half of things and so Satanic/Luciferian religions focus on that part... some of their teachings being very focused on self-discipline (Self-Mastery) and that includes mastering the dark aspects of ourselves... including the chaotic and painful aspects that we don't want to conquer.. our fears... our anger and our lust... etc.
If you look at the Order of The Nine Angles... they have practices which focus on running 7 miles in a matter of 30 mins or so. I don't remember the exact time frame.
As with a person with a terminal or chronic illness. These people are indeed weak in one fashion but if they are fighting the illness and rising above it, then they are strong. The Adversary only gives us what we can handle as an obstacle... that we may rise above it. Thus a person with HIV can in one way of thinking... be given a blessing.
You should read
Liber Azazel. That speaks about strength and weakness in depth.