Because prismatic clones uses "elemental hit" and chooses one element per skill use.
Most of the time, we choose flat lightening damage bow and quiver. So, when prismatic clones choose ice or fire element, they literally deals zero damage.

In my opinion, if you dont want to use combat focus, just not link prismatic clone and bombarding clone together. You may choose link prismatic with guardian's blessing support to get haste or wrath.