[lmm]

>50 years ago it was very reasonable to think that we'd have worked out how to make a fusion reactor too.

Most of the reason we haven't is economic; modern fission reactor designs are far more efficient than even the most optimistic estimates of 50 years ago.

[pja]

Most of the reason we haven't is because it turns out to be really, really hard to contain a high energy plasma long enough to get useful levels of fusion out of it. We still haven't effectively solved the 'how do we efficiently extract the generated energy' problem either.

Perhaps these problems will be solved in the future, but so far we've spent billions and billions of $ with (relatively) little to show for it in terms of output.