Itemizing deductions was once routine for homeowners. It has become a minority practice, and the reason is arithmetic between two competing options rather than a change in what is deductible.
The choice is either-or, not both
A filer subtracts either a fixed standard deduction or the total of qualifying itemized deductions from income, and must pick whichever is larger.
Itemizing therefore only helps to the extent the total exceeds the standard amount. The first portion produces no benefit at all because it merely replaces what was already available.
This is why a deductible expense can be genuinely incurred and still reduce nobody's tax, which is a point that is regularly misunderstood.
Raising the standard deduction raised the hurdle
When the fixed amount is increased substantially, the threshold that itemized deductions must clear rises with it.
Households whose deductions previously exceeded a lower standard amount now fall below a higher one without anything about their circumstances changing.
The effect is largest for those who were only modestly above the old threshold, which describes a very large group of homeowners.
Caps on individual deductions compounded the shift
Limits on how much state and local tax may be deducted, and on the mortgage balance against which interest qualifies, reduce the totals people can assemble.
Those caps bind hardest in high-cost areas, which is precisely where the deductions were previously largest.
Combining a higher hurdle with a smaller achievable total moved many filers across the line in a single step.
The mortgage interest deduction lost much of its reach
Interest is front-loaded in a mortgage, so the deduction is largest in the early years and declines steadily thereafter.
A borrower well into a loan term may have interest well below what would be needed to justify itemizing, even on a substantial property.
The deduction still exists and still matters at the margin, but it no longer determines the filing approach for most owners.
Bunching is the response for those near the line
Filers close to the threshold can concentrate discretionary deductible expenses, particularly charitable giving, into alternate years.
Two years of giving in one tax year may clear the hurdle once, with the standard deduction claimed in the intervening year, producing a better result than splitting evenly.
Thresholds, caps and eligible categories are set by legislation and are revised periodically, so the current rules and a professional view govern any real decision.