What are two reasons constitutions are incomplete guides?

Answer 1

IF you comparatively read the United States Constitution and any federal or State statute section, you will note the stark differences.

The language contained in the former is a arcane and esoteric, and comparatively vague. By contrast, a well-written federal or State statute section will be prefaced with a definitional cross-references section, the language will be verbose and precise, and each individual subparagraph is designed to cover any reasonably-forecast eventuality pertinent to its execution. Any common-law judicial opinion will also be verbose enough to leave comparatively little question as to its contextual preciseness.

The other is that Constitutions, in and of themselves by their own texts and by no external writing, are intended to be "incomplete guides". To really understand U.S. Constitutional law, one really needs to read at least the "landmark" SCOTUS opinions arising from and creating the substance of U.S. Constitutional law, written over the last 235 years, and not just the seven articles and twenty-seven Amendments alone. It's the whole point of Marbury v. Madison and the specific constitutional supremacy and personality of the Supreme Court.