JuicyAds has been around long enough to be a known quantity, and it stays popular because it's approachable. The self-serve dashboard is simple, the marketplace lists individual sites and zones so you can hand-pick exactly where banners run, and approvals are quick - useful when you're testing creatives without committing big spend.
It isn't the place for the absolute highest-tier video inventory, but as a second or third network to diversify display spend it's reliable and transparent. Smaller publishers in particular tend to find it the least intimidating way into paid adult traffic.