The big difference is that when they first considered it they only had one slot pair compared to the three they have now. That said, Sydney is expected to run overnight in both directions which allows the scope for the A380 within Haneda's slots. The challenge will be the daytime parking which undermines utilisation and runs into the need to agree to a special arrangement with the airport. In 2019, the bigger problem was utilisation. That's less of a problem now given that the A380s are fully depreciated and they're not rushing to push fly them to the US.For context, QF wanted a380 to replace their 747 ops to HND after the 747 retirement. They couldn't work out a way to cleanly do it as it involved HND lifting some restrictions and hence we're at 330s.
There's been discussion before about a380 into HND. But without changing the status quo on HND restrictions it boils down to:
Bad landing times (dumping 300+ pax at 11pm with a very tight window for public transport seems ill advised)
Red eye Late night departure (~1am or so)