1. Couldn't public research simply be crowding out private research?
If you're a company and you can use research someone else paid for ... why wouldn't you? Once that model is established, what incentive do you have to spend above your peers?
2. Is the finding stable across time and different countries?
Public funding of research grew enormously in the 20th century alongside the growth of the state and in reaction to various wars (hot and cold).
What did the mix look like in the 19th century? The 18th? What did it look like in Qing China or in late Republican Rome? What differs between Venice at its peak and the glory days of Islamic civilisation?
3. Is much research simply more expensive now?
Requiring funds that only super-profitable companies and governments can muster?
4. How does research vary between industries and problems?
Some kinds of research are more difficult or expensive than others. Is that why there's a surfeit of software companies appearing in the private sector while almost all biomedical companies grow out of government and university laboratories? How does a small business invent a supercollider?
I guess if I combine all these questions, my underlying question is:
5. Is the current configuration causal or just a surface feature of the current background environment, from which it is problematic to extract generalist statements meant to hold true for all time?
My own suspicion is that some kinds of intellectual output are just not very highly demanded in a general market. Patronage, whether by medieval nobility or modern states, is a form of demand that has given us a lot of public goods.