How does "Ending Mortgage Principal" enter into paly when the instructions for completeing Form 1098 does not even metion it.

[Opus 17]

If your beginning or ending balance is more than $750,000, then the mortgage interest deduction is limited.  Also, if your total mortgage loans includes both acquisition debt and equity debt, then the beginning and ending balances are used in the formula to determine the percentage of the interest that is deductible.  

So even though the information is not required to be on the 1098, TurboTax requires it for those calculations.

If you don’t have either of those situations, then the program probably doesn’t need to ask, so there may be a bug in the implementation of the new rules, or it was a design choice to ask everyone.