I suspect that part of it besides being closer to the source and possibly also the mix of demon/angel power in their blood, also the parentage might play a role. Leah as the daughter of Diablo herself might have had a higher basic power level as most that would be descending from a lesser demon or angel. So many reasons it was a pity that Leah was killed off so soon from what she could have brought from lore perspective.
Not sure about this though, but were most of those nephalims considered to be gods not direct descendants of Lillith and Inarius (Both quite higher ups in the power rankings for their respective factions) or was it never mentioned?