Ok, I'm a DRk bagholder so please take my view with this in mind.  IMHO, these are the three main reasons.

Privacy.  Despite what the trolls spew every day nobody has managed to trace a darksend transaction.  If they have, the bounty remains unclaimed.  I.e   The anon works.

Adoption. IntstantX and generally faster transaction times has meant that Darkcoin has been adopted by the gambling industry to a significant degree.  Directbet found that more bets were placed in DRK than in BTC within weeks of starting to accept them.  There is a similar story at other gambling sites.   Separately, more and more dark markets are adopting DRK.  Overall, late 2014 and 2015 saw a huge number of merchants start to accept DRK and there is no sign of this slowing down.

Trust  Darkcoin is developed and marketed by a team who pride themselves on their integrity.  The lead Dev, Evan, won the first crypto Proof of Honour award*.  Many of the rest of the dev team (myself included) use their real names and have significant industry experience and achievements behind them.  Development is fast paced and (sometimes too fast!) innovation is constantly on the agenda.  Trolls and FUDsters will talk about the instamine in the early days of DRK.  This did indeed happen.  However few of those coins are in the hands of the dev team now despite claims to the contrary.  

* It is worth noting that one of the more vocal anon coin rivals to Darkcoin was disqualified from this award for vote rigging.