The low level roulette is broken by design. It's not randomized, it just sticks you in the first dungeon that needs people. Unfortunately, that means that the lower the dungeon the more likely it will come up. Sastasha has the highest chance of coming up, due to the fact that everyone 15+ has access to it. The higher level the dungeon, the less likely you will get it, because there are far less people who are high enough level for it.