It's really, really complicated, since nearly all R&D came from huge government funding and that means politics.

A lot of it is that it came out of weapons research, and u-235 and Pu-239 were well understood from weapons research.

[edit] Also, plutonium fast breeder reactors looked very attractive in the '50s since they could use U-238, with just Plutonium feedstock, and also produced material usable for weapons. It turned out that 1) it's Not That Easy and 2) we don't need more atomic weapons.