I'll try and answer seriously, with some non-exclusive options, in no particular order:

• Feeling ashamed, mostly because they're realizing they're either wrong or sounding stupid.
• Not wanting controversial stuff related to them to be "saved" for others to find and use for scrutiny.
• Honest mistakes (wrong community, thread, etc.)
• The post becomes a cesspool in the comments.
• Other personal reasons (feeling threatened, wanting a clean inbox, question got answered and they don't care about historic purposes, etc.)

These are just my guesses though, and I try and not delete anything personally. I'm aware that anything I put on the internet will be immortalized, and that the healthiest thing I can do is own both my mistakes and my opinions, even if I'm convinced of my stupidity or ignorance at a later time. I'm only a human after all, and doomed to talk before I think. Best I can do is to learn as much as possible from it, and hope that others can also learn from my actions.