"There has been Elite Four members who mono Dark-types, but is the only type lacking for a Gym. Do you think there is a reason behind this and how do you feel about this?"
Well, the reason why they didn't make a Dark-Type gym is that it's less challenging(It can only deal normal damage) than other Types of Gym and it has less pure dark-type pokemon and half and secondary dark-type, they can use Skuntank, Tyranitar, Drapion, Sneasel, Weavile, etc. Just like Jasmine's pokemon, she has 2 Magnemites (Electric-Steel Type) and Steelix (Steel-Ground Type).
I really, really like to see a Dark-Type Gym for future Pokemon games, but it takes a long time to design a whole interior of the gym and ideas on which dark-type pokemon to use to make it more challenging like the other Gyms
I almost forgot that I've made userbars for the club :D
Here the links:
For Kanto: http://www.pokecommunity.com/album.php?albumid=8152
For Johto: http://www.pokecommunity.com/album.php?albumid=8153
For Hoenn: http://www.pokecommunity.com/album.php?albumid=8151
The rest are still in the making