Mainly because they believe it will stay valuable. Which is what backs all stores of value - gold and diamonds are definitely not bought for their practical app

Mainly because they believe it will stay valuable. Which is what backs all stores of value - gold and diamonds are definitely not bought for their practical applications. Now why exactly people believe it is and will stay a good store of value is more complicated, but mostly a mix of trust in how miner self interest will keep the keys safe and general desirability of online currency