I don't find it surprising, just from barriers to adoption: "Wuffs programs take longer for a programmer to write, as they have to explicitly annotate their programs with proofs of safety" is a hard sell (even if it has obvious value) and "you have to learn and integrate yet another language just for parsing files" is a hard sell too.

Which isn't to say that it shouldn't be adopted (having not used it I really don't know), just that it's not surprising that it's having difficulty gaining traction.