The better you are at hiding transaction details, the more impossible it becomes to audit the chain and say with certainty how many coins are in existence. Even bitcoin has a history of near death experiences with infinite inflation bugs, can you imagine how hard that would be to prove or disprove when the transaction details are unknown? Also, many privacy coins are cpu / memory intensive. If you have a resource intensive blockchain to manage and little incentive to run a node yourself, then scaling it to millions or billions of users becomes a nightmare.

A determined user can make any transaction private on any chain by hand coding a series of transactions (to avoid wallet fingerprinting), coin mixing, structuring payments, use decoy change addresses, use tor for broadcast and so on. Privacy coins try to abstract away the hard work, keeping the transactions secret while perhaps introducing an undetectable inflation bug.

Personally, I have a soft spot for privacy-first projects (but not monero) because they put the "crypto" back into "cryptocurrencies". But dash takes an interesting approach, it allows people to hide transactions in plain sight. This will become more evident when usernames go live because although transactions will be transparent, there is no information on-chain as to who paid who.