Energy intensity (and the cost thereof) are a rate limiting factor for thermal desalination:[0]
"Thermal processes are robust, durable and, once in operation, require less maintenance. However, these processes, mainly MSF [Multi-Stage Flash], require large amount of energy for their operation. In fact, up to 50 percent of the operating cost may be attributed to providing energy for the operation of the plant. As a result, many thermal plants are constructed in conjunction with power plants to utilize exhausted steam from the latter. The associated costs of high-energy consumption can have particularly strong restraining effects on small and medium sized plants. However, thermal processes can utilize low grade, low cost source of heat, which would otherwise be lost and discarded to the environment like stack gases, cooling water streams or low pressure exhaust steam. Heat from solar energy can also be used in such processes."
>It surprises me that we haven't worked out a more efficient solar distillation process for desalination.
I'm often surprised at how long it takes to move from discovery to implementation and adoption. The photovoltaic effect was discovered in 1839.[1] Yet 180 years later solar is only 2.4% of the world's installed electricity capacity; transportation and heating/cooling are still overwhelmingly gas/oil.
On an optimistic note, that means humanity probably has tonnes of undeveloped science and engineering that has been discovered but not yet built. It would be infinitely worse if we had already exhausted our collective capacity for invention, yet still had not solved the problems and existential threats we face.
[0] https://pdfs.semanticscholar.org/3f8c/147635ef7dae3050075c2c...