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[vinca BK.]

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[ranochigo]

The capacity of Bitcoin is limited; 1vMB per block. People would be able to choose the fees; paying more if they need a fast confirmation and less if they're willing to wait.

Allowing the user choose their fees allows the fee market to function where people would pay more (in terms of fee rates) for miners to include their transaction in a block. Having fixed fees wouldn't be beneficial at all, people would be able to spam the network at a lower cost as the cost of spamming it remains approximately constant as opposed to the current structure where the attacker has to compete with the rest of the network as well. Miners would also have no preference over most of the transactions as the fee rates are the same, some users could be waiting ages for a confirmation even if they are willing to pay more for it to be confirmed.

[NeuroticFish]

I am surprised that the fees are not fixed, are not the cryptocurrencies decentralized? So who controls these fees and why?

Exactly because it's decentralized the fees are not fixed. This way supply and demand rules come in. So it's the demand that somewhat dictates the fees.
When there are too many transactions to be mined, the ones offering bigger fee will be mined faster. And from here, some will offer more and others will prefer to wait longer.
Keep in mind that the fee calculation is related to the size in (virtual) bytes of the transaction and not the amount or value of money sent.
Also keep in mind that the quantity (total size) of transactions allowed in a mined block is limited.