The lack of investment in this field is absolutely ludicrous given the potential payoff. I suspect some amount of lobbying by entrenched power and fuel interests that would be rendered obsolete, but I think more blame can be placed on that awful meme that brainwashed a generation of scientists and engineers into dismissing fusion as a boondoggle:
"Fusion energy is 20 years away and always will be."
This shows how much damage a superficial catchy meme can do if it manages to slip in and override more nuanced and informed thinking. It's why I distrust catchy sloganish rules in other fields too, such as "premature optimization is the root of all evil" (leads to erooM's law in software) or "never roll your own cryptography" (discourages people from learning about how to create secure systems), etc.