I think until 2015 there was already a major exchange that started the business. Before that Bitcoin mostly traded peer to peer, because there wasn't another way to gain Bitcoin except mining. It wasn't much popular back in 2011, there weren't many propels to buy it was well. And not much altcoin was there. When altcoin started development then slowly exchange is coming as well. Think simply, no demand means no opportunity to exchange. When demand increasing exchanges increasing as well.