Wide adoption, protocol scaling issues and lack of consensus on what to do in order to improve the situation.
The fees work kind of as an auction on getting one's transactions included in the next block by miners. If the lightning network gets finished and works as intended it will likely free up plenty of block space on-chain thus bringing the fees down again.
Wo1f covered it pretty well. You could send a transaction with a very low fee but it would likely take a very, very long time to actually get completed as the miners are always going after the biggest prize. The higher the fee you set the closer to the front of the line for transaction time you get.