[Suicide Junkie]

Well, there could be a number of reasons...
1) You don't actually have any defenses.
- you came from a storm system or a chain of nasties.
- you came through an ally's territory.
2) You have a special, alternate defense plan.
- A black hole system, plus ion cannon frigates.
- A star destroyer.
3) Your defenses are not mobile.
- Lots of sats, fighters, mines, and bases rather than ships.
- Your defenses are planet based, with a warppoint in orbit.