I don’t think the question should be “why was pre-modern era speech and prose so ornate?”, but rather, “why is modern speech and prose so austere?” I believe the primary answer to that question is that we are bombarded with an accelerating stream of words into our ears and eyes and flash now wins over substance.
When something is rare, care is taken to polish and cherish it. Receiving a letter or hearing a speech would be a relatively rare event in the 18th century, so recipients cherished those words and senders took time to deliver a polished product.
Compare that to the mass of words modern citizens are assaulted with each and every day. Speed and attention-getting flash takes precedence over slow-constructed substance when vying for the attention of a word-weary population.
Taking the cue from advertising, contemporaries must grab listener’s attention quickly and to-the-point, or risk losing their audience.
Which ad will sell more product to modern consumers , 19th century’s, *“Coca-Cola is a Delightful, Palatable and Healthful Beverage that Relieves Fatigue and is Indispensable for Business and Professional Men, Students, Wheelmen, Athletes. It relieves Mental and Physical Exhaustion and is the Favorite Drink for Ladies when Thirsty, Weary, Despondent” * , or 21st Century Nike’s, “Just Do It”? Modern people would die of thirst before they got to the end of the old Coke ad.
If a website doesn’t load and grab your attention within 3 seconds, you’ll “bounce” out and Google a site that grabs you in 2 seconds.
Get your breaking news out first and your viewership will increase, while the news agency that takes time to verify facts and spell-check will languish.
Presidents no longer have homey fireside chats, they Tweet.
In days gone by, words were rare and lobsters were plentiful, so even prisoners spoke well and ate lobster. Today words are cheap and lobsters are depleted, so people talk in Netspeak and take out a second mortgage for Lobster Newberg. LOL.