Because there is a deduction limit of $10,000 for property taxes, state and local taxes, and vehicle registration so if the amount you entered is greater than $10,000 per the form instructions $10,000 would be entered.
https://www.irs.gov/pub/irs-pdf/f1040sa.pdf
https://ttlc.intuit.com/questions/4482394-how-will-tax-reform-affect-my-2018-federal-tax-return