[DennisP]

It's because light water reactors use thermal (i.e. slowed down) neutrons, which can only fission U235, and that's 0.7% of natural uranium. We enrich the fuel to at least 2% U235 and that's mainly what the reactor can use.

The reactor also gets about a third of its energy by fissioning plutonium, which is formed when U238 absorbs a neutron.

Without reprocessing, a light water reactor can't even fission all the U235, because some of the waste products of fission reactions are strong neutron absorbers. (If you're in France you can reprocess the waste to remove the fission products.)

Molten salt reactors remove most of the fission products continuously. Some MSRs, like Terrestrial Energy's, are still thermal reactors using uranium, so they're about as efficient as France.

Some MSR designs use fast neutrons, which are able to fission more plutonium along with other transuranics and U238. Others use thorium fuel, which can be completely fissioned by thermal neutrons. (First thorium absorbs a neutron and becomes U233, then that fissions. Heavier non-fissile isotopes aren't created in the first place.)