The first problem is inertia. You need a critical mass of users before anyone will bother to accept a particular payment processor, but users don't sign up if no one accepts the payment service. This is the reason why people still use Paypal.
The second problem is fraud. If someone commits fraud over a matter of $0.20, what amount of resources can you really afford to spend investigating it? A micropayment processor that thoroughly investigates fraud will have to charge the sellers a large chunk of the payments they process, but one that doesn't will be bankrupted by fraudsters.
Crypto currencies have some potential to break this by changing who eats the loss in the case of fraud. If it isn't the payment processor (or there is no payment processor) then micro-payments become viable. You still have problems with e.g. malicious software stealing bitcoins, but they're different problems that don't break micro-payments.
The first problem is inertia. You need a critical mass of users before anyone will bother to accept a particular payment processor, but users don't sign up if no one accepts the payment service. This is the reason why people still use Paypal.
The second problem is fraud. If someone commits fraud over a matter of $0.20, what amount of resources can you really afford to spend investigating it? A micropayment processor that thoroughly investigates fraud will have to charge the sellers a large chunk of the payments they process, but one that doesn't will be bankrupted by fraudsters.
Crypto currencies have some potential to break this by changing who eats the loss in the case of fraud. If it isn't the payment processor (or there is no payment processor) then micro-payments become viable. You still have problems with e.g. malicious software stealing bitcoins, but they're different problems that don't break micro-payments.