Beyond it being expensive to replicate once, for observational studies and clinical trials, you need many repetitions because of statistical variability. That gets expensive in a hurry.
We felt the preclinical stage was more important to validate, as much of the research is based on academic studies with over 60% failure rate for replication (see http://www.nature.com/nature/journal/v483/n7391/full/483531a...)
By clinical trials, a lot of the pharma companies have already done initial validation, so extra validation is redundant. We hope a reproducibility system to validate preclinical research can help patient groups, foundations, and industry better identify reproducible oncology targets, and hence the focus on preclinical studies (which in turn are less expensive).