I also can't vote by mail just because I want to. I have to be out of town on official business to "qualify" for a mail in ballot. Being on vacation isn't even an apparently accepted reason (on mobile but I'll get a link)
Edit: Link to PA absentee ballot application: http://www.dosimages.pa.gov/pdf/AbsenteeBallotApplication.pd...
Relevant parts:
Section A: I declare that I am eligible to vote absentee at the forthcoming primary or election since I expect that my duties, occupation or business will require me to be absent from the municipality of my residence on the day of the primary or election for the reason stated below; and that all of the information which I have listed on this absentee ballot application is true and correct.
Section B: I declare that I am eligible to vote absentee at the forthcoming primary or election due to the illness or physical disability stated below; that the information required to be listed pertaining to my attending physician is correctly stated herein and that all other information that I have listed on this absentee ballot application is true and correct.