A few months ago there was a similar thread that you may find interesting. Here's an excerpt from my answer that pertains to your questions:

There isn't a really good answer as to why titles were so long. They could just as easily ask us why our titles now are so short. That being said, there are a few considerations that could help us to understand why they did it so differently than us. I'm just building on what someone else here pointed out so well, that the novel had to find ways to promote itself in a time before it asserted its cultural dominance in the 19th century. Also, your question should really be about pre-modern (generally 17th century) and 18th century books, because the 19th century had generally ended the practice of these long-winded titles. As I pointed out elsewhere in the thread, the serialized novel in the 19th century could not afford to have such long titles, since space in the newspaper or magazine was very precious. The 19th century was also the beginning of standardized printing practices that allowed for a "trade binding": a book cover with art and promotional material. I always use the beautiful example of the Hetzel editions of Jules Verne to show how very commercial the book cover business had become by the mid 19th century. The 18th century hadn't yet adopted these advertising practices, so the interest of books had to be explained in plain language on the first page. People at the time fully understood that the long titles were basically secondary description around the title itself. No one referred to Robinson Crusoe as "The Life and Strange Surprizing Adventures of Robinson Crusoe, Of York, Mariner etc. etc". Even when the title was something short(ish) like "Candide, ou l'optimisme" ("Candide, or Optimism") from 1759, no one would have thought to call Voltaire's book "Optimism," as they all knew the real title was "Candide."

I hope that helps you. Let me know if you have any more questions!