Bitcoin is the oldest of all the cryptocurrencies available today and also has the biggest market capitalization amongst all other cryptocurrencies available today. It has the advantages of network effect and proven security. Here are the main features which makes bitcoin different from altcoins.
Bitcoin has the biggest lead as a store of value and has existed 8 years without any failure. The security provided by Bitcoin is unbeatable and is far more than its younger counterparts.
Bitcoin is easily accessible and has a better number of exchanges, more merchants and more supported hardware and software than altcoins. Bitcoin has much larger volumes in the market and hence more liquid as compared to altcoins. The developer ecosystem is the largest and there are more software and hardware implementations than any altcoin. There is a lot of intellect, dedication and creativity around this cryptocurrency which is going to make it more useful.
Altcoins have to compete with a number of unique features of Bitcoin when they want to compete with it. There is a big user base of Bitcoin, a great functional development team and efficient mining operation. They also have to compete with a large ecosystem of startups, entrepreneurs and open source projects supporting Bitcoin system.
Being the oldest running cryptocurrency and with the largest market cap means that there would be a number of attacks on the Bitcoin system for years. So it is the most tested and safest if we consider this on terms of technical hacking. The processing power is large in Bitcoin system and this means that there is a minimum feasibility of a majority attack (51%).
Well, this does not advice you not to invest in altcoins at all. And a portfolio of cryptocurrencies need to diversified anyway. The bulk of your portfolio should be proven cryptocurrencies like Bitcoin and Ethereum. Others can be altcoins that you believe have a good future.