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.