Many potential neurotoxins are circulating in our blood, including those from endogenous sources such as metabolites or proteins, or exogenous ones such as xenobiotics ingested in the diet or otherwise acquired from the environment.
I would speculate that other areas of the body haven't developed similar protections because their structures are not as vulnerable to stuff that's floating around in your bloodstream.
The eyes have a similar barrier. I think the current belief is that your body wants to make sure that your T-Cells cannot accidentally nuke your brain and eyes. Those don't grow back and are vitally important, so they have to be protected from the "Nuke it from space" approach of the immune system.
Nit: I’d rephrase to say that your body has no opinions about T-cells. But people who had proto-BBBs survived better than those who didn’t, AND T-cells would nuke the central nervous system from space.
https://fluidsbarrierscns.biomedcentral.com/articles/10.1186...
I would speculate that other areas of the body haven't developed similar protections because their structures are not as vulnerable to stuff that's floating around in your bloodstream.