For three reasons:
- it resembles the fasts of Ramadan that you are making up (i.e. the fasts of Ramadan are fasted one after the other with no break in between)
- it absolves you of your responsibility to make up those fasts sooner
- it is a safer approach to take because you don't know what is going to happen to you tomorrow; perhaps you will arrive in the evening as a person alive, or you may arrive as a dead man.
Adapted words of Ibn al-Uthaimeen (Sharh al-Mumti')