It was incorrect of me to imply that "evolution" is an entity with a plan. Allow me to take this in another direction.
First, we can reduce the problem further. In terms of individuals, each individual may frame his curiosity for the sake of bettering his peers. After all, intensely curious researchers tend to make great discoveries. In terms of ancestry, this is analogous to being the alpha male, in order to win life and pass on his genes (by "win life," I mean get dates, eat food, and otherwise survive to the best of each individual's ability). Thus understood, no human is actually curious for the sake of being curious, but for the sake of 1. being better than other humans, which generally leads to 2. surviving. There is a more in-depth discussion of this here. (pardon me, I can't get the link to embed)
By the way, a shameless example would be my posting this to regain some degree of pride. Do I actually want to answers to my questions, or do I just want to be better than other people? It's hard to say, but I lean to the latter.
As for the phenomenon of diminishing curiosity, it can be attributed to the fact that only one person can be the best. In sports, that's ok - the Superbowl is held every year and you get many chances to be the best. In science, something awesome can only be discovered once. There'd be no point in trying to discover the nature of lightning, because Ben Franklin already did that. He already won.
If I want to win, I have to discover something of my own (or do something awesome). Curiosity is cultured by the scientist to win, to beat his peers. The most curious scientist probably works the hardest, and has the greatest chance of winning.
And now, you'll try to post a response that critiques my own, but not for the sake of a greater truth. It'd be for the sake of beating me. Humans aren't very mature, are they?
A thought experiment for you:
Imagine that tomorrow, the folks at LHC find the Higgs boson and presumably solve the universe by discovering and proving the Theory of Everything.
Would your curiosity about it be diminished?
For me, the answer is yes, and I suspect you'll find the same of yourself. If you're not convinced, you only need to look towards history. Right now, you aren't curious as to the chemical composition of water. If it was the year 1700, you probably would be. However, since the concept of H20 has entered into our collective cognition, we aren't curious about it anymore. It is not so egregious to think the same would happen with the Theory of Everything. The question remains: why does this happen?
I think it's got to do with an evolutionary mechanism for the collective progress of humanity. A limit on natural curiosities per human streamlines the discovery process. It works like this: a curiosity for the unknown burns in many humans, but only for things that no human knows. This burning drive allows a large amount of minds to actively work towards the truth.
Now, once one mind figures it out, the flame is extinguished. After all, it would be inefficient for the flame to keep on burning. It would be inefficient for the same large amount of people to continue slaving away towards a hopeful discovery of the answer when a much easier way presents itself: learning it from those who know.There is no need to reinvent the wheel, as they say.
Instead, it's more advantageous for people to occupy themselves with other unsolved curiosities. The more people working on a particular problem, the higher the chance the problem will be solved, and the sooner humanity can move on and solve the next problem.
The mechanism is simple. We identify with the people who actually know by proxy. It is enough that they are human. We consider "humanity" to have solved the problem, and we (most of us) consider ourselves to be a part of "humanity." In that way, it's almost like we ourselves know the answer.