My pet theory is that a good amount of the early coins are effectively "dead" with either the keys lost, brain wallets forgotten or the back-up USB keys long since formatted (or you know, you got arrested and the encrypted wallet is seized as evidence and revealing the key or otherwise moving the coins would basically be an admission of guilt)
It's easy to set up triple redundant cold storage and promptly place multiple paper backups in different safety deposit boxes now when the price is rounding $200; but how many people went through this amount of effort when bitcoin was still considered a joke or nifty social/economic experiment and traded below $4 or even $.04
Even better; how many people stored them in an insecure manner and have since lost them to a scam/virus/offline attack.
Those are all risks that come with using Bitcoin. I'm sure with time we'll see lots of improvements. For now a good way to store coins is by using paper wallets: http://www.bitaddress.org/ -you can print them yourself using any printer. Some say your keys shouldn't even be on any computer until you're ready to use them.
But for the rest Bitcoin is like cash. You lose the key you lose the cash.
Most times when one person "loses" cash another person "finds" it, leaving the money supply unchanged. When a private key is lost all the coins in that wallet are lost effectively forever. If this happens with large accounts the potential for it to effect the overall economy cannot be ignored.
Well with my paper wallet analogy, if you lose your private key, someone else will find it as well. But anyway, there are enough divisible Bitcoin units that this doesn't pose itself as being a huge issue.
When the network will be hit hard by large amounts of bitcoins trapped behind lost private keys, I'm sure there will be some implementation changes to the Bitcoin software in order to free up the coins and make them go into circulation (to be mined).
For example if an address hasn't been touched for 20 years (death? lost key?).
