The best know name in cryptocurrency is still Bitcoin. Bitcoin was the first coin started in 2009, and is still the biggest cryptocurrency. All other are seen alternative coins or altcoins.
Over the years many thousands of altcoin projects have been started and also many of these projects have also been abandoned, leaving coins and tokens in the hands of people without any value or use-ability.
All these starting projects, when they came available on an exchange were traded against the big brother Bitcoin. This way the small altcoin projects did strengthen the position of Bitcoin ,even if they created value inside their own coin environment. You should also consider that many developers when making a use case for a crypto coin, they also accept bitcoin as second option. This way they don’t just build their own coin , but also …. again Bitcoin.
When all exchanges use bitcoin in their trade pairs, it will make bitcoin more valuable, or the altcoins relative less valuable when there come more alts every week.
This where just some thoughts on how and why Bitcoin is the most dominant cryptocurrency. And this will not change until all coins are mainly traded in fiat currencies.