Running the Bitcoin network securely is expensive. Currently, about 95% of the cost is covered by issuance of new Bitcoin, but the amount of remaining Bitcoin decays exponentially (currently it’s down to the last 5%).

The incentive design of Bitcoin assumed that it would be used as cash, so that transaction fees could take over as the main source of revenue for the miners that secure the network. As a “store of value”, it doesn’t generate enough transaction fees to secure the network.