Technically, SATA SSD's need to fill this spot as a cheaper alternative but with their prices being just as expensive (often only 5% cheaper) as m.2. If SATA had a price range in the 0.30 EUro/TB, it will have been a great alternative to HDD based storage.
And you can go really crazy with bifurcation > 4/4/4/4x and then converting all those new m.2 slots to 6 SATA (ASM controller for cheap and work great). Plop, 24 SATA ports for 4W power draw.
But nobody is going to pay for SATA SSD's when they can just buy m.2 for the same price. So the result is that the SATA SSD market is "kind of dying", and manufactures look at it like 2.5" drives got looked at from HDD manufactures.