Why might a pilot or flight attendant not benefit from these tax deductions?

To be able to write off flight crew tax deductions, it is necessary to itemize.  Think of all of the flight crew expenses as one big itemized deduction.  While they might be big, they might not exceed the standard deduction unless the pilot or flight attendant has other deductions to help exceed it.  Typically, if a pilot or flight attendant owns a house and has a mortgage, the mortgage interest alone will make it beneficial to itemize.  Adding flight crew expenses on top of that leads to a significant reduction in a pilot or flight attendant's tax liability.

Another factor is something called the 2% limit.  If a crewmember's employee expenses do not exceed 2% of his or her adjusted gross income, the crewmember will not benefit from employee expenses.

Lastly, if a crewmember falls under the AMT (Alternative Minimum Tax), no tax deductions will reduce his or her tax liability.