It's been around the longest, has the most infrastructure built for it, a first mover advantage, and a strong network effect.
Exchanges don't sell bitcoins. They provide a platform where traders who have bitcoin can offer it for sale, and traders who have fiat currency can place bids. Any time the two meet, a trade happens. Exchanges hold balances in various currencies (fiat and crypto) for their customers, which customers can withdraw. There are also OTC markets like localbitcoins and Bisq, where coins are escrowed but fiat currency changes hands without an intermediary - either in person or via cash deposit to a bank account.