Why the network fee is changing suddenly from one day to the next? One day transaction fee costs ~20USD, the next day it's already ~30USD. Why is that happening? I always thought that the fee is constant.
A Bitcoin block can contain only 1 mb of data. A transactiom averages around 255 bytes. A block is built about every 10 minutes and the blockchain forces the difficulty to do so. So per hour, only around 6mb worth of transactions can pass, however there's a much higher demand due to Bitcoin's popularity. If only X transaction can fit in a block but five times X would like to do a transaction, only the highest bidding X number of transaction will proceed. At this point it's basic supply and demand understanding.