Why would they remove Bitcoin from their list? it doesn't matter how strong Bitcoin is, the reason why Coinbase exists it's because Bitcoin exists it was only made for Bitcoin.
I don't see there's a Bitcoin belong their list, you might mean it's Bitcoin cash (BCH).
BCH, ETC, XLM, and XRP, are the coins they stop supporting, but I think XRP is still useful when it comes to sending funds because it has a very low fee compared to other altcoins. The rest deserve to be delisted.
It might Coinbase choose a coin that frequently uses by their users, they earn profit through transferring fund and the reason of stop supporting them because their company didn't earn from this coin.