Whatever happened with the research of about 20 years ago, that found post exertion resulted in massively higher accumulation of lactic acid in CFS sufferers than the general population?
With the studies by Myhill, Booth and McLaren-Howard and Fluge & Mella, we now have a good idea of why this excessive lactic acid build-up appears in ME/CFS patients after exertion.
The issues appear obvious. There was interest but no funding, just like most of ME and CFS research. Lactic acid was probably not exciting enough to warrant funding compared to other research, and when only a small number of studies can be funded per year it took time for the research to accumulate. Now, as @Hip says, we have a lot more evidence. That makes the research more exciting. Which means we should see a lot more of it.