After entering income and having tax showing as due why does it not drop after taking the standard deduction?

The standard deduction is the default.  So, it is automatically applied from the very beginning - i.e. when you enter your income, the program automatically takes into account the standard deduction when showing the tax due or refund amount.  Then, when you go to the Deductions section, if you enter more itemized deductions then the standard deduction, the program will switch to itemized and give you the larger benefit.  

It shows you might be using the desktop software.  You can always switch to the Forms Mode in desktop (top corner that looks like a stack of papers) and see your Form 1040 to see the deduction being applied to your taxable income.