Bitcoin is the most dominant among all cryptocurrencies because obviously it is the pioneering, has the largest community, adoption, usage and legalization depending on every country. Altcoins are not that known by normal people they only heard of Bitcoin most especially here in my country. Bitcoin is the first tradable crypto into our local fiat currency here in my country and followed by Ethereum just recently. Popularity is one of the factors affecting the dominance of Bitcoin and it's value is just a bonus to it's supporters.