It has fallen to the lowest level of all time compared to 37.7 percent because the price of Altcoin has increased. This is the dominant index of bitcoin, which keeps this domination of Bitcoin above the global cryptocurrency market.
The main reason for this is adoption of increasing popularity and alternative cryptocurrencies. It has decreased from 90% to 37.7% of the Bitcoin Management Index of 2017. It has been at least 50 times with the previous year, in which ripple has shown a 330 times increase in value. It has increased in some currencies like ethereum, wave and light cone.
SOURCE - https://www.ccn.com/bitcoin-dominance-index-drops-to-all-time-low-at-37-7-rise-of-altcoins/