Because you told TT it was a distribution. A rollover or a Roth conversion is not a distribution.
From the instructions for form 8880:
Line 4; Don’t include any of the following.
• Distributions not taxable as the result of a rollover or a trustee-to trustee transfer.
• Distributions that are taxable as the result of an in-plan rollover to your
designated Roth account.
• Distributions from your eligible retirement plan (other than a Roth IRA)
rolled over or converted to your Roth IRA.
