Any ideas why impacted cerumen causes so much pain?

Ear. If the wax is hard or if there is an infection behind it it hurts a lot. Get it checked out as soon as possible.
Pressure on eardrum. In this instance pain is not usually related to infection but to pressure from the impaction on the eardrum.

Related Questions

Any ideas why impacted cerumen (accumulated earwax) causes a lot of pain?

Inflammation. Likely inflammation or infection. The ear wax (cerumen) likely has gotten wet. You now probably have an external otitis (swimmer's ear). See an ENT to remove ear wax and treat. Read more...