bitcoin slowly went down this year, from almost 20k USD last December and went down at a significant price
what might be the reason why bitcoin's price is going down?
what are the major things that can greatly influence bitcoin?
answers would be fully appreciated

I think negative news can affect bitcoin prices, as many sellers become panicked, and bitcoin prices are falling, as bitcoin prices are set by the market.
The major factor is the demand of any market that affects the price of a product and in case of crypto world the coins are also demand dependent. The price will go high if the demand goes high and the price will go down when the demand goes down. This is the major factor controlling by the encouragement and discouragement of the people, if people are encouraging then price will go high and vice versa.