Why do C++17 structured bindings not use { }?

The National Bodies from Spain and US have proposed to change back to the {} syntax because (P0488R0):

The “structured bindings” proposal originally used
braces “{}” to delimit binding identifiers. Those
delimiters were changed to brackets “[]” under the
assertion that they didn’t introduce any syntactic
problem. However, they turned out to introduce
syntactic ambiguity with attributes and lambdas. In
the light of various suggested fixes, it appears the
original syntax is more adequate.

Therefore, there still remains the possibility of ending up having the original syntax for C++17 (which I strongly believe is preferred by most users).

Update from this trip report:

The original proposal for decomposition declarations used the syntax auto {a, b, c}; that was changed at the last meeting to auto [a, b, c]. This change was fairly controversial, and several comments asked to change it back to {} (while others encouraged keeping the []). There are technical arguments on both sides (the [] syntax can conflict with attributes once you start allowing nested decompositions; the {} syntax can conflict with uniform initialization if you throw Concepts into the mix and allow using a concept-name instead of auto), so in the end it’s largely a matter of taste. The clang implementers did report that they tried both, and found the ambiguities to be easier to work around with []. In the end, there was no consensus for a change, so the status quo ([] syntax) remains.