My pet theory is that previous worlds have often created some sort of Necropolis or undead society where enough sentient undead exist to grow a church of Hel and thus when she took the bet she thought that there would be a reduced but still usable pool of beings to draw clerics from. However I would say that the self aware fantasy theme in this world raised adventuring to an unexpectedly high career choice and thus all undead clerics have too short an unlife expectancy to create such a stable society for her.