Why is the market capitalization dominance indicator BTC.D for Bitcoin so important???

Because as the bull market gradually enters its later stage, Bitcoin's share in the entire cryptocurrency market, which is BTC.D, will slowly decline. This does not mean that Bitcoin is not rising, but rather that its gains may not keep up with other cryptocurrencies, especially altcoins. This phenomenon is quite common because at the end of a bull market, funds often start flowing from Bitcoin to altcoins—people are more willing to take a chance on doubling their investments.

This is actually the golden window for retail investors to make money. In the second half of every bull market, altcoin performances are often the most vigorous, while Bitcoin's market share often fails to reach the highs of the previous bull market. This means that although Bitcoin is still rising, it is the altcoins that are truly experiencing explosive growth.