Using common airframe for three variants was probably the major flaw of F-35 program. I think they were concerned about the design costs. They made it work but

Using common airframe for three variants was probably the major flaw of F-35 program. I think they were concerned about the design costs. They made it work but it was expensive to get working. Plus, the electronics and software were the really expensive pieces. It would have probably been cheaper and better to design two planes, one conventional (F-35A, F-35C) and one vertical take-off. Then save money by using common parts and the same avionics.