Because Coinbase is a very easy place to use if you want to get involved in Crypto quick, this is something which allows for a random investor to get involved without too much of a technical barrier to entry. They're also getting the funding due to the amount of fees that they're able to collect from these same investors who don't know the tricks to be able to same money in fees (such as using GDAX instead of buying on coinbase) Investors like that they're a trusted name in crypto, they collect high fees, and they're continuing to get more and more customers. It's a godsend for investors.