Quintessential and essential are two different words with two different meanings, that is why. Your mistake is thinking those two words mean the same thing. They do not.
'Essential' is when something is required, necessary or important. 'Quintessential' is when something is a perfect representation of a thing.
Flour is essential when making pancake batter. Not quintessential.
Rolex is the quintessential luxury timepiece. Not essential.
So, when you see the word 'quintessential', either in this game or elsewhere, it's because someone is trying to convey that specific idea to you - that something is quintessential. It's not because they are trying to be difficult, or complicate things unnecessarily when they could have just said, 'essential'.
"...well there's your problem."
But seriously, though, just chalk this up to English not being your native language. You have good command of it, but some things (like old English or less-common words) might still trip you up, which is completely OK. It's just all the more reason to continue studying the language.
