See this FAQ
Because your income was a little less in 2019, and/or your deductions were a little greater. The standard deduction is more in 2019, so that has something to do with it. See this website for some interesting info:
https://www.fool.com/taxes/2018/11/18/its-official-the-2019-standard-deduction-is-gettin.aspx
Of course, if you mean a smaller refund for 2019, then it is the other way around despite the increased standard deduction. If you are on Social Security your income increased by 2.8% over 2018, and the Part B premium taken from your SSA check went up just $1.50/mo from $134 to $135.50. So net SSA income went up some in 2019. If you are employed, then something may have changed to reduce your gross income such as greater health insurance premiums or other deductions from your check and tax withholding. You could compare 2018 and 2019 W-2 forms to determine that. If your income at work was not less, then the only other cause that I can think of is a difference in tax rates/brackets for 2019 vs 2018.
And the IRS changed the withholding tables so you got more in your checks during the year instead of a big refund. Or there are other tax changes. And any dependents you have are a year older. If you made more income that can reduce some credits you got like the EIC.
Here's a good FAQ on why your refund might be less than last year