Bitcoin has a networkeffect.
First off its the best known cryptocurrency. Most people have heard about Bitcoin before but like Ethereum for example is less known by the masses. I think this is kind of a positive feedback loop. Media writes about popular topic because it get's more clicks. The more people read about it the more well known it becomes. So they keep writing about it. Nobody cares about an unkown altcoin, they rather write unimportant topics like: "Who is Satoshi Nakamoto?". Because people love stories like these. It makes a viral article and since people don't pay anymore for news it's about advertisement income.
This network effect also makes it usable as a currency, because to be a good currency it has to be universal accepted. If no merchants are accepting cryptocurrencies you can't do anything with it. It's still pretty rare to see merchants accept Bitcoin but for altcoins this is even worse. Especially online there is a trend of more webshops accepting Bitcoin.
Another reason is the high security of Bitcoin. It's slow and not very scalable but proof of work is very secure. And the more nodes and miners there are the more secure the blockchain becomes. So the fact that Bitcoin is the biggest kind of make it the most secure blockchain. (Side note, it's only secure when it's decentralized. For example if 1 big firm would mine >50% off all Bitcoins then it's not very secure. )