[lhorie]

Sort of the same reason why `if (true) let a = 1` is a syntax error (and the error message says why: "Lexical declaration cannot appear in a single-statement context")

The rationale here is that the variable goes out of scope right away due to the implicit block, so they made it a syntax error so that you don't waste time pulling your hair out debugging issues downstream from this line without realizing this scope-related obscurity. (This also has potential security implications e.g. if you think you're shadowing a variable from a parent scope but you're not)