There are many factors. The minivan itself is dying, and it's almost a "perfect" car for many use cases - can hold 8, can get car-like mileage, can hold as much as an SUV, is easier to get into.

But they're dying because people ain't buying, because the costs are so high and those with the money to buy new vehicles don't like them.

There is NOTHING preventing small SUVs that hold 5 people and cargo, get really good mileage, and are cheap(ish) - but they don't exist used because they don't exist new.

(It's sad, because I'm buying a new minivan, and I could not make the PHEV Pacifica make sense no matter how I finagled the financial numbers, the Sienna is simply unavailable at reasonable trim levels, and nobody makes an EV minivan.)