Because the apostrophe as used in English is a punctuation mark, not a letter, and especially not a modifier letter. The author argues that any name ought to be

Because the apostrophe as used in English is a punctuation mark, not a letter, and especially not a modifier letter. The author argues that any name ought to be matched by anything in \w, and we should avoid a punctuation character for that reason, but he doesn't mention other punctuation marks like the hyphen that also commonly occur in names.