Simple, bitcoin has gain the trust of the community due to it years of continuous profitable performance, it has always revolves in a circle as it always recover after it decline in price meanwhile since most altcoins are just duplicate of already existing alts, it usually get replace with a new and improved project which kills the former. Meaning, altcoins can't be relied on but bitcoin can be relied on since we only have one bitcoin.

The cryptocurrency market is still influence by emotions meaning most Investors are just emotionally motivated and have no prior knowledge of investing therefore they fud on any negative news and since the altcoin market has more of this type of Investor, it surfers the most.