Why are documents like the US Declaration of Independence capitalized differently from modern writing?

In 18th and 19th century English it was fairly common to capitalize nouns (though not pronouns, unless you were talking about God or Jesus, in which case you might see "Him" or "He," as you still see sometimes today). Modern German still capitalizes all nouns (and I believe adjectives used as substantives, too, like, "The Meek shall inherit the Earth," though someone better at German might correct me).

Something I find interesting is that we capitalize adjectives when they're derived from a place-name, but German doesn't. So we'll say "an English beer" or something, and in German you'll see "ein englisches Bier," with "beer" capitalized but not "English." Same with adjectives derived from someone's name, like "Socratic" versus "sokratisch."

Here's a relevant portion from linguist David Crystal's Cambridge Encyclopedia of the English Language:

By the 17th century, the practice had extended to titles (Sir, Lady), forms of address (Father, Mistris), and personified nouns (Nature). Emphasized words and phrases would also attract a capital. By the beginning of the 18th century, the influence of Continental books had caused this practice to be extended still further (e.g. to the names of the branches of knowledge), and it was not long before some writers began using a capital for any noun that they felt to be important. Books appeared in which all or most nouns were given an initial capital (as is done systematically in modern German) - perhaps for aesthetic reasons, or perhaps because printers were uncertain about which nouns to capitalize, and so capitalized them all.

The fashion was at its height in the later 17th century, and continued into the 18th. The manuscripts of Butler, Traherne, Swift, and Pope are full of initial capitals. However, the later 18th-century grammarians were not amused by this apparent lack of discipline in the written language. In their view, the proliferation of capitals was unnecessary, and causing the loss of a useful potential distinction. Their rules brought a dramatic reduction in the types of noun permitted to take a capital letter.