WTF? You can use steroids and still remain skinny, even emaciated. It depends on the dosing of steroids and especially on the amount of food and its content. You do not become buff just from ingesting oral steroids and/or injecting them in the muscle. The enhanced protein sythesis must be feeded by enormous amounts of food. Otherwise, you will just make muscles stronger without significant weight gain. Power to weight ratio is crucial in distance events. Furthermore, even if you do not gain tremendous amount of strength on steroids if you are on a reasonable diet, you do get the benefit of much faster recovery.
Couple that with EPO and HGH, and you have all the advantages you need. The pitfal is using detectable steroids, which are harder to mask, unless they are designed specifically for you in the lab. Kisorio was stupid to use common steroids. Had he been using just HGH and EPO, and followed simple protocol, they would never detect him. Just like thousands other elite endurance athletes.
The poorer the country, the less likely it is that even the highest profile athletest will be tested out of competition. Thus they are safe from being busted, if they are not stupid. Kisorio obviously was.