I guess RCTs are complicated to do because you often can't generate homogeneous control and treatment groups, so you are either forced to laboriously measure every relevant aspect of each group to standardise. Or to otherwise invent clever ways to do your treatment that ensures that most of the effects from unavoidable differences cancel out.
The thing is, these complication doesn't explain why nobody overcame them until Kremer, Duflo e. al. started their experiments in the 1990's. Their work appears to be a simple adaption of methods from other fields to studies in developmental economics, not any sort of technological development. (This is one of the earliest papers cited in the motivation provided by Nobel foundation: https://pubs.aeaweb.org/doi/pdfplus/10.1257/app.1.1.112 it does some linear regression at the most)
With creation of new technology ruled out as the blocker for performing the experiment, you are basically left with internal and external sociological explanations.
The thing is, these complication doesn't explain why nobody overcame them until Kremer, Duflo e. al. started their experiments in the 1990's. Their work appears to be a simple adaption of methods from other fields to studies in developmental economics, not any sort of technological development. (This is one of the earliest papers cited in the motivation provided by Nobel foundation: https://pubs.aeaweb.org/doi/pdfplus/10.1257/app.1.1.112 it does some linear regression at the most)
With creation of new technology ruled out as the blocker for performing the experiment, you are basically left with internal and external sociological explanations.