The butterflies are adults that do not need to grow, they just need energy, so they normally eat only nectar. They have a mouth adapted for sucking liquids, which could not be used to eat solid food, like pollen.

On the other hand, the bees collect both nectar and pollen. Nectar to provide energy for themselves (which they dehydrate into honey, which can be stored for a long time, unlike fresh nectar), and pollen, which is rich in proteins, to feed their larvae, which need to grow into adults.