This may be anecdotal evidence at best, but I found that when the Cracked Clusters were displayed in Levelling Roulette, the queue times for DPS became much faster (even displaying Healer in Need at one point). Most dungeons took like 10 minutes max as a DPS after that, but soon went back up.

This is on Chaos btw, and even now, the maximum I had to wait for a SB dungeon is 20 minutes, and Leveling roulette 10 minutes max, and that was recently. As such, I feel the best way to get people to tank/healer is to make it worth more to play and far better rewards than DPS to make it appealing.