Anyone know how or why certain exchanges only choose to carry certain coins? An exchange like coinbase only offers BTC, LTC and ETH. I don't even see the point of trading there.
It's depends on the exchange's marketing strategy and also their budget, having more coins costs more money. For example coinbase is not an traditional exchange they only focus on some coins and offer primarily buying and selling functions via fiat. While other more conventional exchanges like bittrex or poloniex offer wide variety of BTC-altcoin markets, they make their primary profit from the fees taken from exchanging btc for altcoins.