They were banned because the "classic" function-argument-tuple model and tuple conversion rules caused type system problems, particularly because every T was implicitly convertible to (label: T) and back. We've since shored up the argument label model, and various people have discussed limiting the tuple conversion rules to be less unpredictable and less straining on the type checker. If we solve that problem, then there's no technical reason remaining for single-element tuples to be banned.
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