Considering that Tyrael and Imperius have always squabbled over helping humanity, a new power struggle might be underway in Heaven, keeping those sympathetic to humanity from aiding them.
It's also unlikely that players can expect Angelic assistance against this returning threat, as what few lore notes have been released so far indicate the High Heavens have cut themselves off from Sanctuary and are not answering mankind's desperate pleas for help. It seems extremely probable that the early part of the story will deal with the player investigating Lilith's return, trying to understand what she is up to. The story of the game is taking place over at least five different regions: Scosglen, Fractured Peaks, Dry Steppes, Hawezar, and Kejhistan. Lilith is exceptionally powerful, as the daughter of a Prime Evil and has a seductive pull on all she encounters, gathering followers to her side as her simple presence is enough to unearth base desires in stoic hearts. Lilith's return will have devastating effects, as new cults will spring up in worship, while monsters are confirmed to be roaming the world, including massive packs of ferocious, bestial Goatmen. Still, Rathma went into hiding and his death was never confirmed, so it seems likely the simplest answer is the correct one and Rathma, for whatever reason, has decided to bring his mother home. Rathma also usually chose to take more neutral approach in the war between Heaven and Hell, striving to maintain the balance between the different forces. Now, technically it could be another Nephalem, as Lilith is the mother of the entire Nephalem race (and by extension, all of humanity). Source: Blizzard Entertainment (Image credit: Source: Blizzard Entertainment)