You are right, it is both clear and difficult. It is difficult because first you need funding and political will. IDUs and sex workers are rarely considered priority populations from the point of view of governments, but in concentrated HIV epidemics such as there are in Central Asia, these are exactly the populations that need to be prioritized.
Next, the populations themselves are difficult to reach. Because of stigma and discrimination, as well as the criminalization of sex work and drug use, people who engage in these activities are hidden. To reach them, we have to draw them out to drop-in centers that attract them with needed services, or find them with outreach workers who know and understand the population.