There are mainly two reasons why Bitcoin remains as the most popular crypto-currency out there. These are:
1. Bitcoin was the original crypto-currency. Other alts such as ETH and BCH are at least partially based on Bitcoin.
2. Most of the trading for the other crypto-currencies (perhaps with the exception of BCH) comes from trading versus BTC. Alt vs fiat trade volumes are quite low.
Yeah thats right. besides that because bitcoin is the first cryptocurrency and makes bitcoin more popular and firstly adopted by many people and that's what makes bitcoin a much larger user than altcoin users. other cryptocurrency is just alternative