Why would my standard deduction be $5,974 instead of $12,400?

[Hal_Al]

The usual reason is you are a dependent.  A dependent's standard deduction is limited to $1100 or $350 plus his earned income  (but no more than $12,400).  This is to be sure that he pays some tax on any unearned income he has.  That is, the standard deduction is limited to $1100 against unearned income (interest, dividends,  capital gains, unemployment, etc). 

Taxable scholarships are counted as earned income for the standard deduction, but not anything else (IRA contributions, earned income credit, etc)

[JohnW152]

The most likely reason is that you've reported in TurboTax that someone else could claim you as a dependent on their return, so your standard deduction is limited.

According to IRS Publication 17, Your Federal Income Tax,

The standard deduction for an individual who can be claimed as a dependent on another person's tax return is generally limited to the greater of:

• $1,100, or
• The individual's earned income for the year plus $350 (but not more than the regular standard deduction amount, generally $12,400).

However, if the individual is 65 or older or blind, the standard deduction may be higher.

[VolvoGirl]

Another reason is if you are filing Married filing Separate and your spouse is itemizing on their return.