Network Congestion on the Blockchain Competitively Raises Fees
The main reason for high bitcoin miner fees is supply and demand. The bitcoin block size is 1MB, which means that miners can only confirm 1MB worth of transactions for each block (one every ten minutes). If the number of transactions waiting to confirm exceeds what can fit in 1 block, bitcoin miners choose to confirm the transactions with the highest bitcoin miner fees.
The year 2017 was a good example of high miner fees. For all of 2017, the number of bitcoin transactions people wanted to make every 10 minutes exceeded what the 1MB block size could handle. As a result, miner fees skyrocketed and Larger Transactions Cost More to Send
Generally, bitcoin transaction fees are directly proportional to the size in bytes of your transaction.
Sending a bitcoin transaction is a lot like sending mail through the postal service. If you're sending a small letter, it is quick and inexpensive. However, if you're trying to send a large package, shipment will take longer and be more expensive.