That's because its built in part on XMLDSig, a genius idea to sign active content that can redefine its own semantics as it's being signed/verified. It's a triumph of ideology over common sense.
The first book that came out on XMLDSig, "Secure XML: The New Syntax for Signatures and Encryption", written by the chair of the working group, spent over half of its 500 pages wrestling with canonicalization, and even then it read more as a 250-page problem statement than a solution.
And that was before you got into deliberately malicious content that actively subverts the signature process.
