Bitcoin's value is constantly increasing. Why is the price of bitcoin increasing over time?
It is because the buyers of bitcoin are increasing. The spread of bitcoin is continue and with the passage of time more and more people know about it and they invest their money in bitcoin so with that demand the price of bitcoin increase.
The demand is growing and therefore the price is rising. The more people buy bitcoin, the higher the price. After all, we know that the total number of bitcoins is limited.