Bitcoin has gained momentum over the period of time and now user base has grown much bigger since inception. However, the focus is concentrated on the speculation part of it instead of the utility value of the same. As of now, the price is determined based on the law of demand and supply but still, it is highly dependent on the speculative market instead of the demand for actual ground level use of Bitcoin.
The cost of the majority of cryptocurrencies, including Bitcoin, is based on trust in it, therefore, having lost confidence in early 2018, the cost of Bitcoin began to rapidly decline.
Given this, I think that speculation and manipulation greatly affect the cost.