Quick bans are a council tool to override lengthy processes to remove unhealthy Mons, and requires quite high % votes from the council (>=2/3). It had enough support to go on the tiering slate, and then enough votes to get the QB. They went though process and volc got banned. So what if 'nothing new' was added to it: the meta has changed, and people felt that it's presence was enough to let it go. And it's not like it's been completely off the radar, as there were complaints about it in the pre home meta, so it's comes as no surprise to see it being booted.
If people are still salty about the volc QB, they can let the council know, and especially during surveys. And then, if there is enough support, they can do a retest. Like with Cinderace and Melmetal back in IoA gen 8 meta. You can still have your QB and suspect, just we get to see a volc less meta. Also, who cares whether it's a new introduction or not. See IoA Cinderace QB.
To be honest, Volcs probably been high on the test list for a while. It's pretty much been one of the most potent setup sweepers for a while. In BW ou, they decided to ban gems over volc, Gen 8 nearly saw it suspected during the IoA, but it was short lived, but remained on people's radar. This gen, it got better at matchup fishing, and we are finally done. It's a very strong Mon, and a real matchup fish, but, finally, it's been shown the door. Again, if there is enough support, or we see tera banned, or we get DLC, expect it back.