Does the common man think of "cryptocurrency", or does he think of "bitcoin"? If in that persons head, "bitcoin" is the same as "cryptocurrency", then it will be really hard for a competitor.
Google is a nice example. "I will google it for you". Nobody says "I will bing that", or, sadly for other search engines, "I will search the web". We say "I will google it". In our head, google is the search engine, and the search engine is google. So if bitcoin is in that same ballpark, good luck to the competitors.
But I'm not saying other cryptocurrencies can't bring anything new to the table, such as Ethereum or Monero. But bitcoin definitely has the brand positioning advantage.
So one reason BTC is worth more (speaking of entire market cap here) than, say, LTC, is because it is accepted more places.
Another reason is that BTC is perceived (rightly or wrongly) as more stable than other crypto currencies.
But all those legitimate reasons are amplified dramatically right now by speculation.
Most importantly, the Coin that has the highest expense to "mine" is the most secure. To attack Bitcoin by out pacing the current miners, it would require millions of dollars in hardware which could just as easily be used to legitimately acquire Bitcoin by mining.
However second place is easily attacked, as the case with BCH, the bitcoin attempted fork. Miners are using their hardware to primarily mine Bitcoin, paying for the hardware, then occasionally attacking BCH, to break it's adjustment algorithm.
Bitcoin is a global cryptocurrency. Sure it has a first-mover advantage but so does Google. Today someone searches with google, tomorrow they may switch to Bing. Can't the same be said of Bitcoin and say Litecoin / Dogecoin / whatever? So why is Bitcoin worth more than Litecoin let's say? Both are accepted by Coinbase etc.