If we paid 100% of what we owed for 2019 on our quarterly estimates for 2020, why do we need a form 2210?

It's 110% if your adjusted gross income (AGI) for 2019 was more than $150,000 ($75,000 if your 2019 filing status is married filing separately).  In any case, you must have paid those quarterly taxes in proportion to your income for each quarter.  In fact, you can even get an underpayment penalty if you're getting refund.

You need to file a form 2210 so the IRS won't charge you a penalty for underpaying your 2020 taxes. This shows them that you made a good-faith attempt to pay them by paying the same as you owed for 2019. The form 2210 is required to be filed by the IRS.