Well. There is a number of reasons for that. First of all like you said, Bitcoin is the first ever cryptocurrency to be created. It is the longest running and most trusted. Naturally people would flock to it for this reason.

Moreover, Bitcoin is the most successful too. Its marketcap is the highest of all the cryptocurrencies. It continues to dominate the market and most new comers to the "crypto world" would commit themselves to the best cryptocurrency.

Moreover, Bitcoin is not like other altcoins in the fact that Bitcoin can actually be used in real life. It is a currency and an asset. Other altcoins are only good for trading and investing. They have no real use in life.

These are a few of the reasons why is bitcoin the most popular cryptocurrency.