That's a good question. I think it's because since Katrina and Victor-Cindy weren't a single, continuous system and were rather related and non-continuous ones. When Katrina degenerated into a remnant low, the BoM likely treated as the end of Katrina’s life since the LLC dissipated, which is why when that RL became a tropical low again (what would become Victor–Cindy), it got a new name, and hence considered a different system.

That’s why they have different names and are recorded as separate systems, even though they're related, which is why they don't hold the record for longest-lived instead of Freddy. That's just what I think tho, I could be wrong.

They weren't a continuous system and are recognized as separate systems iirc

When the low level of a cyclone get disrupted it stops being considered a storm, usually in this cases only the high level circulation and unestability remains, it can generate a new cyclone.