Why do I need to report the ending mortgage principle when it is not a required reporting item on my 1098

Yes, you are receiving the request for your ending mortgage principal balance due to the change in tax law related to the mortgage interest deduction.  There are now different mortgage interest deduction limits based upon when the mortgage was originated and what it was used for, so portions of a mortgage listed may include interest that is not deductible.  Therefore, the ending mortgage balance is needed to determine the deductible portion of the interest being entered.  You can find your ending mortgage balance information on your statements or by checking with your mortgage company.

Here is more information about the new tax laws and the provisions for now being able to deduct mortgage interest - click on the section called "mortgage interest and home equity loans" :

https://ttlc.intuit.com/replies/6971774