Everyone has heard how some people claim that Bitcoin doesn't scale because it's blocks are too small, but the real reason for that problem is that our Internet connection doesn't have enough bandwidth to allow average users to process thousands of transactions per second. So, what does that have to do with privacy coins? Well, their transactions tend to weight much more than those of traditional coins, because of all those complex signature algorithms that create that privacy - hence, privacy coins are even less scalable than Bitcoin. Of course you could increase their block size, but it would cause node centralization, which would quickly defeat any privacy. So, private coins right now can't become a widespread currency, they can only serve as store of value.