I think the biggest reason for Bitcoins huge dominance is that people that don't even know what crypto is invest in it whenever they see it on the news. You don't really see the news talking about how Ethereum has went up 30% in the last month. At least not on the regular news stations. Whenever the news says that Bitcoin has returned 100% this year, there will be brand new people coming in to the market and investing in Bitcoin.
This is one of those cases in which we do not know what came first, after all I could make the argument that most people prefer to put their money in bitcoin because it already has a huge market cap and that is what causes its popularity among the media.
But regardless of the reasons behind the popularity of bitcoin it does not seem like this is going to change and independently of what you may think of bitcoin it is clear that bitcoin is by far the best investment this market can offer.