Because we don't want bitcoin to go mainstream until everyone's grandmothers can easily store their bitcoins.... the current solutions are either unsafe or way too nerdy for mass-adoption.
+1
I was helping someone set up a wallet (Mycelium on Android), and since I insisted, I walked him through creating a backup of the key, written down on paper. I have no idea what happened to this backup, and if I wasn't there, the average person may not have even made one, or lost it somewhere.
People are not used to the importance of protecting money in this way:
If you lose your checkbook, you get the bank to put a hold on it, and issue you a new number, if necessary.
If someone forges a check, the bank will reverse it.
If someone steals your credit card, you get a new one, and you are covered for most fraudulent transactions.
If you have gold coins, you keep them locked in a safe.
When you carry cash, you keep a limited supply, and you keep it close.
Bitcoin has an entirely new security model, which non-geeks are not familiar with. We need a solution for the masses before it's safe to release Bitcoin to ordinary people.
As it becomes more mainstream, I think more businesses will get involved to make the payment process easier, to the point where it's just an NFC tap, similar to paypass on a credit card.