Why tax for a life insurance 1099-R beneficiary?

My relative passed and I elected a lump distribution on the life insurance payment. The 1099-R listed about a fifth of the gross distribution as 2a Taxable Amount, which I think was the profit made above the insured amount. (It's categorized as "D4" which I take to be: Death, annuity payment from non-qualified annuities that may be subject to tax under section 1411.) I got a separate small 1099-INT.

My question: If the life insurance was in form of a non-qualified annuity, why do I owe taxes on this amount? Wasn't my father taxed on this amount during his lifetime since it was profit above the base amount?

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