Well bitcoin transaction fees are very high which some of us cannot bear with it imagine that you are just send $10 dollars worth of bitcoin you have to pay $20 in fees just to confirm very fast and currently the minimal transaction fee is 900 satoshi per byte so, in average sized transactions you have to 0.002 ($33 in today's rate) in fees just to confirm your transaction and because of that some investors and holders were move to bitcoin cash and other cryptocurrencies for lower transaction fees and fast confirmation so, that's why bitcoin dominance were getting lower and lower.