Simple, Bitcoin is a good investment that's why more people went crazy about it in the last couple of years. Besides, most altcoins are pump-and-dump scheme much volatile that bitcoin is. Although bitcoin's dominance was challenge by ETH and Bitcoin Cash, they never succeeded because bitcoin has the lion share of the support by the community.
One reason is probably because bitcoin is a pioneer in blockchain technology. In addition, because the amount of supply is very limited, most people tend to choose bitcoin for their investment
It very obvious that up till this moments Bitcoin is still many investors choice and there are know much challenges poss by the likes of Ethereum and bitcoincash or even Ripple as this time.
Yes, bitcoin is the first crypto, it has maintained that lead till now, no doubt about that, and whether because of it limited supply, it could have been an added advantages. However, Ethereum, Bitcoincash, EOS, Tron etc, has actually given the crypto economy a boost, bringing in a good competitive atmosphere to the crypto market.