Fusion has been tough because it has been a never-ending series of problems and clever solutions. None of the big designs have been breakthrough to the brass ring. By limiting the projects, we limit the number of people thinking about fusion. We are limiting ourselves; it is no wonder fusion power is still decades away.
I think it has been a case of trying to tdentify a laboratory method which preents some promise of steady progress toward a controlled, sustainable reaction. (I don't think it's a matter of more money.)
I don't see how you can know in advance whether a method will work until you spend some time with it. That's why certain methods (e.g. diodes) were abandoned.
It appears to be a problem whose solution can only be obtained through painstaking laboratory evolution. I personally am confident that the class of nuclear engineers working this problem are bringing the physics along as quickly as knowledge will allow.