Why did it take so long for people to discover new elements?

Now you are asking for people to make up stories based on personal opinions taken from historical accounts, and very few people know enough history to form any opinion at all. My opinion is that historical people have mostly been farmers who had little need to learn anything more than farming methods. Almost everything you consider to be a basic necessity was discovered during or after WW2.

Science is not self supporting, so the first concern of a scientist has to be to avoid discovering anything that embarrasses his patron. Success is measured by approval of other scientists, which assures continued support, use of exotic equipment, and professional contacts, so a scientist must also avoid discovering anything that his peers disagree with. The only reason science ever advances at all is that old scientists die and are replaced by young scientists with different patrons and different peers. On that basis, science was supported by the Catholic Church for most of the last two thousand years, and currently by the U.S. Department of Defense and food manufacturers.