"Why haven't other coaches, agents, athletes, and sponsors who've seen Block hanging around said something before now?"
Athlete on the circuit know who were dirty but they don't speak out because exposing dirty athletes would ruin the sport for all. So often on forums like letsrun you here statements like "they're all dirty." This is just not true. I would say the majority of athletes are honest guys trying to do things the right way. But they understand track fans and society in general are fatalistic and would prefer to condemn all instead of giving people the benefit of the doubt. So they would prefer to keep quite, do their own thing honestly and let the chips fall where they may.