Because Darkcoin is currently under threat by Monero and Quazar coin. Once these coins get their proper wallet, things should take off for them as well.
Ring signature based coins appear to have massive scalability problems. It might be very well possible they just can't cope with the bloat to go mainstream.
That's irrelevant
I was watching the Poloniex trollbox yesterday. They were pumping the Monero price by reference to the recent Darkcoin rise. Then Bytecoin was added. The Trollbox went mad, Monero got dumped and Bytecoin got picked up, again by reference to Darkcoin.
The problem for these coins is that they are now borrowing the Darkcoin brand recognition and brand value. That means they inherently do not have any sustainable brand value themselves, at the moment.
Also, Bitcoin is way ahead of the field. The amount of VC money going into creating merchant services and new products is phenomenal. Darkcoin is based off the Bitcoin code, probably via Litecoin adaptations. This means that all Darkcoin has to do is demonstrate competence in the quest for becoming anonymous digital cash with mass appeal or adoption and there is an argument for it to be added to any merchant platform that already hosts Bitcoin. The effort to integrate will not be trivial, but it could be put on the table as an option.
The same cannot be said of any technology that has a different code base to Bitcoin. I bought some MRO to better understand the mechanics, economics and community. Looks like I'm bag holding at the moment.