Why did those aircraft disappear?
Perhaps because there has never been any market.
You answered your own question.
Then why did those clever people, supported by hundreds of a betters, make a market forecast demonstrating the "existence" of the market and launched the A380?
Because of market failure
all across the board in the years prior to & after 2000 when the A380 was launched :
1. airport congestion, delays, noise & fuel pollution all externalized
to the public, public is not reacting enough (sheeple)
2. new engine technology in the process hid from Airbus, says Leahy : thus imperfect & asymmetric information
use of frames on some expected routes, like CX or BA flying WBs 30' to 4h apart LHR-HKG or LHR-JFK
4. by the same token, unforeseen rise of the ME3, specially EK : the ME3 have a skybridge or sky train UK-DXB-Oz
5. unforeseen shift from dedicated cargo frames to passenger frames-bellies
6. unforeseen success scale of the 773, took over the 744.
7. unforeseen 2001 crisis
& the slump thereafter.
8. abysmal bugs & delays of Airbus' own making, including not making the A388 the optimized platform
9. traditionnal airlines shy after all these events
10. No consideration for the majority of the Demand : PAX actually love it & enough go out of their way to fly a VLA
there is more.
And more reasons why the success was also unlikely, Boeing saw that right. They launched the 748i not to sell really, but to cap the A380 price & also Airbus' ability to negociate wider fleet campaigns, a strategic move ( taught at Harvard Biz School as a typical industrial / business case ), paid for by healthy 748F sales.
And later new efficient twin WBs nailed the ageing VLAs.
So the market was there in 2000. And still is : cramming >400 Pax into A330s or 10 abreast 777s points to VLA demand.
In Economics, it doesn't matter what the frame is, but what is their capacity & operation costs.
Thus canning >400 sardines is a VLA
in name or as a dedicated model.
Yet what we know today is markedly different from what was known then to the clever people.
As many here have said, the 773 is in many ways superior to operate than the A380.
But the VLA are not gone actually : technically, the 77X is a VLA.
The "market" in itself is not to blame, but rather their deciders : they played small instead of the Big Picture, with the notable exception of EK who proved Airbus right beyond belief. Whilst less efficient, the 748 is also a remarkable plane, not just as 748F. Interesting that LH & KE ordered both VLAs.