[Difwif]

From my understanding it comes down to part politics, part feasibility. When nuclear power first started becoming popular light water reactors were easier to make and their fuel/byproducts aligned with the production of nuclear weapons. As years went on regulation and inertia did a great job of cementing us into our old ways. Today, advances in (material) science have made producing molten salt reactors more feasible. They also seem more attractive now that the public is generally very adverse to nuclear waste and power plant failures. I'm paraphrasing what was explained to me so someone that knows more please chime in.

Kirk Sorensen has been a major advocate of molten-salt reactors and gave a great Google tech talk on the subject. https://www.youtube.com/watch?v=bbyr7jZOllI