Not only because Btc is the first cryptocurrency, your investment is also very secured with right wallet address (non-custodial). That's the first thing investors look at. However, not all who are first will remain first and not all who are last will remain last. Btc was the first but he remained the first until now. His popularity continues to grow especially Bitcoin payment can contribute to that. And what should old investors invest in, of course that cryptocurrency that has already been established is very low risk. It is also one of the contributors that Bitcoin continues to rise because of more investments, especially since Btc has a nice feature called "scarcity".

There's no other cryptocurrency that has the same value as Bitocin. But there are some coins that almost have the same volatility (has less volatility) with Btc because they also have huge marketcaps. Those coins are in the top 10 cryptocurrencies in coinmarketcap and are best to invest with.