Notice how it takes some time for pilgrims to become butterflies in the time of Lothric, only appearing when the flame is weakened and the darksign appears in the sky. Yet at the close of the age of fire when the Dreg Heap forms, the fire/darksign is weak enough to allow humans to fully ascend to angels.
In Lothric's time, the flame hasn't yet faded enough to where pilgrims can become true angels. I believe the difference between angels and butterflies is how strong the darksign is. With the fading of the fire, the darksign is too weak to contain the spreading darkness. Normally this darkness would be kept in check by the darksign, which was the sign's original purpose going off the description of the ringed knight weapons. Since the pupa/chrysalis-type enemies who resurrect the angels pop out of the pilgrims' back, we can connect angels/butterflies to that spreading darkness.
Pilgrims have roots coming out of their backs and are likely 'seedbeds for spreading darkness' as described in the cleric set. Butterflies are not seen in the Dreg Heap but only in Lothric, so the two paths of evolution seem to be mutually exclusive. We know angels only seem evolve in the far, far future.
