It is basically because of the fear of the people in the cryptocurrency market and community. People are transferring their wealth from altcoin to bitcoin because they think that it is the safe bet in the cryptocurrency market because bitcoin really dictates the trend for the overall market so they don't need to worry about other things. It is like an index fund.
Probably. We all know that the crypto market behaves in strange and bizarre ways. Since 2018, things have proceeded negatively for crypto. There's been lack of interest within altcoins in general, as investors support Bitcoin in every way. This may not last for long, as altcoins are implementing innovative features for the mainstream world to use. There are many cryptocurrencies out there which are faster than Bitcoin itself. Once the pioneer cryptocurrency becomes slow and expensive again, then investors will head back to altcoins.
I believe that Bitcoin's market dominance will always be high for a good reason. After all, Bitcoin's the most secure cryptocurrency to date. No other altcoin, could match Bitcoin in terms of resiliency and censorship-resistance. But of course, this doesn't mean that altcoins will disappear altogether. Only a select few will remain in the space due to their dedicated development and innovation over time. Ethereum is one of those altcoins that will remain behind Bitcoin for years to come. When altcoins rise in price while Bitcoin remains stagnant or bearish on the market, the pioneer cryptocurrency's market dominance will decline. The same thing happens vice versa.
Nonetheless, with the level of growth Bitcoin has experienced over time, its market dominance could reach 90% and above. Before the bull market in 2017, Bitcoin used to have an extremely-high market dominance. Time will tell us how farther will the market reach as institutional investors come into play. Just my thoughts