Cost per GiB is higher and long term reliability is lower in most scenarios.

The failure scenarios for spinning rust tends to work better with large storage arrays as well.

Not all absolutes, but enough of them are true on a common enough basis that spending the extra on SSD's isn't usually worth it.

If you want some real in depth explanations there's probably a datahoarder community somewhere or reddit if you are so inclined.

I can get a 20tb HDD right now for $300. When I am talking about 100s of TBs it makes a difference