You probably needed to update your W-4 form with your employer, if you're not an independent contractor or run your own business. A W-4 is a form that tells your employer how much federal tax to withhold from your paychecks. If you go to your 1040 and compare the lines for federal income tax withheld and they're pretty similiar from this year and last, that's most likely what happened. Nothing you did wrong.

If that's not the case, something must've changed from last year to this year. I'd compare each line from 2018 and 2019 on your 1040s and see what changed significantly (other than your income and tax liability due, since we both know those went up).