Probably because it's more difficult to build infrastructure for. As an example, the Ledger Nano hardware wallet still isn't compatible with Monero but is with

Probably because it's more difficult to build infrastructure for. As an example, the Ledger Nano hardware wallet still isn't compatible with Monero but is with ZCash, despite XMR being older and having a much larger market cap than ZEC. Also Gemini, another leading US exchange, added ZEC recently and Coinbase wanted to catch up.