1. There's not enough time.
Every project always has too many features, too few people to code and test them, and not enough room left in the schedule to finish.
2. It's really not a bug.
Maybe you've heard the phrase, "It's not a bug, it's a feature", misunderstand the feature, test errors, spec changes..
3. It's too risky to fix.
Always it's ofen true, you might make a bug fix that cause other bugs to appear. Under the pressure to release a product under a tight schedule, it might be too risky to change the software.
4. It's just not worth it.
The bug would occur infrequently or appear in little-used features may be dismissed.
5. Ineffective bug reporting.
The tester didn't make a strong enough case that a bug should be fixed. As a result, the bug was misunderstood as not being a bug, was think as not important important enough to delay the product, was thought to be too risky to fix, or was just plain considered to be not worth fixing.