it is simple: coinbase or any other service like it is newbie friendly.
you just sign up there and you can buy/sell/hold/spend/receive bitcoin all in one place without even knowing what bitcoin is, and how it works.
and since people are lazy, they choose these wallets. in comparison bitcoin core is a big pain in the ass for them because you have to download the whole blockchain (70-80GB) and it takes a very long time and you need to understand what bitcoin is how to send a transaction, how to secure your wallet.
besides not everybody is aware of the dangers of using these web wallets, they see a pretty website and pretty promises of security and they believe them until they get hacked like bitfinex and poof all the money is gone.
There are a lot of newbie friendly SPV wallets or API wallets which at least don't control your keys. For example Blockchain.Info wallet, MyCelium or Jaxx. It's very stupid to use custodial services like Coinbase.