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.
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.
You pretty much summed up my opinions as well. I view DASH like I view linux containers. They are taking something NOT designed to be secure and private/anonymous and trying to retroactively bolt on anonymity and security to the existing bitcoin tech. Which is NOT now you secure something. It has to be designed in from the get go not baked in after the fact.
This is the problem with all the so called "secure anonymous" coins I looked at until I read up on Monero and how it works. Security and anonymity is baked in by design, not retrofitted after the fact which never works.
So as far as I can tell Monero is the only one that has this right. Now, again, time will tell if their technology choices actually deliver what they say it will deliver regarding anonymity and security.
But for now it's the only game in town as far as I am concerned.
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.