Why arent bitcoins that solve more difficult problems worth more?
Bitcoins don't solve problems. Bitcoins are given to miners for solving a block as a way of providing an incentive to perform the mining, and as a way to fairly issue the currency to the community.
Or the first bitcoin, that should be worth more. I would rather have one millionth of the first bitcoin or the last bitcoin than one in the middle.
Due to a quirk in the way bitcoin was created, the first 50 bitcoin are unspendable. They will most likely remain forever trapped, unspent, in the first block created.