Certainly a lot of the cost of the project is R&D, but most people would be shocked by how much of aerospace project budgets are driven by quality. E.g., a bolt costs $200 not because it has to go through a new R&D cycle, but because it needs a chain-of-custody, inspections, metal coupons stored, etc.
There's also a huge amount of political risk for a government entity. Politicians will be reluctant to fund another JWST if the first one fails because many will fight it as a waste of money, and the previous failure just bolsters the JWST-opponent's position.
There's also a huge amount of political risk for a government entity. Politicians will be reluctant to fund another JWST if the first one fails because many will fight it as a waste of money, and the previous failure just bolsters the JWST-opponent's position.