The main reason is people sell their cryptos when it reaches to 19k. It dips to 17k and they begin to buy bitcoin and the price increase massively to 18-19k. Traders do not want to risk their fund for the next thousand dollars I guess. It reached 19900 ATH but it is hard to reach 20k.
This scenario is not going to last forever. May be for the next few weeks, the short-traders will be able to perform this cycle. But eventually the long term investors will step in, and remove the additional supply from the market. Once this is done, the supply squeeze will ensure that the exchange rates will breach the 20K resistance level. But even if this happen, there is no guarantee that the prices won't fall back to sub-20K levels. Spikes and corrections are a part of Bitcoin, and we need to get adjusted to it.