The reasons for the fiat standard are that gold serves primarily as a store of value, but it is not effective money across space due to issues of transportability and associated costs. Additionally, gold is not an ideal unit of account. Silver, on the other hand, was easier to divide, which is why both assets were used historically.
The gold standard was a solution for a certain period, but we all know what unfortunately happened during World War I, when more paper gold was issued than actually real gold in the banks existed. The same might happen with Bitcoin by using L2s.
The main reason for the fiat standard, however, is the political coercion behind its use, along with the lack of a better alternative in all three dimensions of the characteristics of money.
