Bitcoin was the first of its kind to come into the mainframe of cryptocurrencies. Altcoins were created and introduced to the world later.
Bitcoin holds almost 70% of the market cap, leaving only a small margin for Altcoins and other Cryptocurrencies. The various kinds of Altcoins circulating in the market are creating unwanted competition for themselves.
New Altcoins are introduced very frequently in the market, and many even die a natural death. Whereas, Bitcoin has stood the test of time.
The ROI that Bitcoin has offered over the years to investors is much higher than any Altcoin has ever yielded. These are the many reasons why Altcoins have not surpassed Bitcoin yet.