Economics.
I hate to tell you, but I am working on fuel rods that are made from something other than zirconium so there is no production of hydrogen possible.
If successful, then it is a “cheap fix” and will put molten salt on the back burner.
Secondly, the problem at Fukushima was the lack of power to the circulating pumps. Today, every nuclear plant in the US is “hardening” their systems with backup generators/pumps in safe buildings away from the plant that can be hooked up quickly to restore power.
Fukushima exposed a flaw in the safety of nuclear power plants. But it is a flaw that is easily fixed without new reactors (that may bring their own new set of issues with them).
one of the principle claims of msr reactors is that they will produce electricity at a fraction of the cost of lowest cost coal. because the resource is cheap and because they don’t require all they layers of protection that a light water reactor requires.