I can tell you my reasons, but it's mainly the mandatory privacy, where every single transaction has to use RingCT and helps to protect the privacy of the whole network.

Zcash's very computing expensive shielded transactions are rarely used and no GUI (and mobile) wallets simply offer them. You can see this in their block explorer: https://explorer.zcha.in/transactions Here is some more information on their own forum: https://forum.z.cash/t/low-percentage-of-total-shielded-zec-... Most exchanges also don't accept shielded Zcash deposits/withdrawals.

DASH's private send is just similar to a bitcoin mixer and takes even more than 24 hours to finish, since the Masternodes have to wait for mixing partners for you. This mixing creates a shitload of transactions on chain, it were around 70 when I tested it lately and that's a huge overhead. That are probably the (main) reasons why it's rarely used and since Chainalysis, Blockseer and all the Bitcoin tracking companies have no troubles to track mixed transactions, like they did with Wanna Cry, I think it's only a matter of time to extend them for DASH with more adoption: https://cointelegraph.com/news/bitcoin-exchange-shapeshift-h...

Summarising optional privacy makes where almost no one uses it, makes you suspect just for opting in for the privacy feature. It's also much easier to track just the few percent of private transactions by design.