I've been a coinbase customer for four and a half years.  Coinbase is under the thumb of financial authorities.  Recently Coinbase acquiesced to an IRS subpoena for customer records.  Coinbase could have appealed, but they gave in.  Coinbase is such a big, fat target that they are probably afraid to add a coin like Monero.  All the bulldogs and retrograde anti-crypto buttheads like Feinstein and company would be on Coinbase like flies on shit.  Coinbase isn't going to add Monero any time soon.