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.
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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.
Speak for yourself, the Pentagon, UK, France, Russia, China, India, Pakistan, Israel, North Korea, possibly others still replacing and/or building tons of those.
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.