Because, if you see that, At the end of 2017 the value of bitcoins reached at the highest position. No one expected such types of growth of bitcoins. It was really unbelievable .It continued at the first two months of 2018. But after few month later and the mid of 2018 it started to decreasing instead of increasing. It is expected after few month later or at the last of 2018 the value of bitcoins will be high position again.
Normally, the whole cryptocurrency market is fully loaded with huge speculation and that makes it easy to see the value start going up pretty high in a short amount of time and then see it go down rapidly before you even know it. For me, we are still in the continuation of no real life usage which obviously has given room for more speculation in this market and as long as that is concerned, we will keep seeing huge reds following huge greens and vice versa.