"Having a great design and potential customers is one thing. But development is hugely expensive, and manufacturers need to be confident in financing it and making a future return on this investment. For the 787, for example, the total development cost was estimated at over $32 billion (as reported by the Seattle Times). Whether it ever sells enough aircraft to break even on this remains to be seen."
https://simpleflying.com/the-business-of-making-a-new-commercial-aircraft-type/