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

There is a lot of reasons why. Panic selling, selling a huge amount of bitcoin because they think it will continuously drop. Country banned bitcoin, some country have the higher population of people using bitcoin and banning have big impact in the price and also the supply and demand.