Historically, Fusion has been bad at predicting when it's going to be ready for prime time, that plus very early fusion experiments turned out to be a lot of duds. Just think of the Cold Fusion hype (on a side note; cold fusion does work, µCF replaces electrons with muons in the hydrogen atom which reduces the radius of the atomic nucleus such that fusion becomes possible at room temperature and far below but generating the necessary muons is a fools game) and the various other nuclear fusion failures.

Fission on the other hand could report a lot of results and success and, at the time, seemed to be infallibly safe.