Because, in prospective studies, the amount of stressor exposure and the occurrence and severity of mental problems in the study population are difficult to predict and both may be only moderate (only few subjects may experience significant adversity and few may develop lasting mental problems), prospective resilience studies typically need large sample sizes to reach adequate power. This is one reason why multi-center studies and international collaboration are vital for resilience research.
Another reason is that resilience mechanisms discovered in one study population need not necessarily operate in other populations. The question of the generalizability of results can only be solved when comparing data from resilience studies world-wide.
Hence, resilience research is laborious and expensive and needs wide-spread support by the research community, by host institutions, funding organizations, stakeholders, and by the public.
However, we believe this effort is worthwhile. Read here about the prevalence of stress-related disorders in industrialized societies and about why research on resilience and disease prevention is a strategic priority of mental health science.